sync bin_text_write and bin_write for empty obj
sync bin_text_write and bin_write for empty obj

refers #876

> about confusing inconsistency in bin_text_write and bin_write. They store 0-length objects in a different way. 1st one write nothing while 2nd one write 4 zero bytes (length). 2nd approach would be better for saving empty strings (no need to store trailing 0 byte), but changing this may make models binary incompatible.
hal3/vowpal_wabbit,zachmayer/vowpal_wabbit,lmatthieu/myvw,deaktator/vowpal_wabbit,martinpopel/vowpal_wabbit,digitalreasoning/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,marko-asplund/vowpal_wabbit,deaktator/vowpal_wabbit,deaktator/vowpal_wabbit,lhoang29/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,pombredanne/vowpal_wabbit,deaktator/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,deaktator/vowpal_wabbit,pombredanne/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,deaktator/vowpal_wabbit,zachmayer/vowpal_wabbit,marko-asplund/vowpal_wabbit,deaktator/vowpal_wabbit,digitalreasoning/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,trufanov-nok/vowpal_wabbit,pombredanne/vowpal_wabbit,pombredanne/vowpal_wabbit,trufanov-nok/vowpal_wabbit,lhoang29/vowpal_wabbit,hal3/vowpal_wabbit,marko-asplund/vowpal_wabbit,trufanov-nok/vowpal_wabbit,lhoang29/vowpal_wabbit,michaelbrundage/vowpal_wabbit,digitalreasoning/vowpal_wabbit,martinpopel/vowpal_wabbit,michaelbrundage/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,zachmayer/vowpal_wabbit,raosudha89/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,danmelamed/vowpal_wabbit,martinpopel/vowpal_wabbit,tzukuoh/vowpal_wabbit,martinpopel/vowpal_wabbit,lhoang29/vowpal_wabbit,lmatthieu/myvw,pombredanne/vowpal_wabbit,digitalreasoning/vowpal_wabbit,digitalreasoning/vowpal_wabbit,martinpopel/vowpal_wabbit,marko-asplund/vowpal_wabbit,raosudha89/vowpal_wabbit,marko-asplund/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,lmatthieu/myvw,danmelamed/vowpal_wabbit,danmelamed/vowpal_wabbit,pombredanne/vowpal_wabbit,tzukuoh/vowpal_wabbit,hal3/vowpal_wabbit,hal3/vowpal_wabbit,raosudha89/vowpal_wabbit,raosudha89/vowpal_wabbit,deaktator/vowpal_wabbit,martinpopel/vowpal_wabbit,lhoang29/vowpal_wabbit,michaelbrundage/vowpal_wabbit,digitalreasoning/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,hal3/vowpal_wabbit,hal3/vowpal_wabbit,trufanov-nok/vowpal_wabbit,michaelbrundage/vowpal_wabbit,digitalreasoning/vowpal_wabbit,zachmayer/vowpal_wabbit,pombredanne/vowpal_wabbit,tzukuoh/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,lhoang29/vowpal_wabbit,tzukuoh/vowpal_wabbit,deaktator/vowpal_wabbit,hal3/vowpal_wabbit,tzukuoh/vowpal_wabbit,hal3/vowpal_wabbit,hal3/vowpal_wabbit,lmatthieu/myvw,jon-morra-zefr/vowpal_wabbit,zachmayer/vowpal_wabbit,martinpopel/vowpal_wabbit,trufanov-nok/vowpal_wabbit,danmelamed/vowpal_wabbit,michaelbrundage/vowpal_wabbit,tzukuoh/vowpal_wabbit,digitalreasoning/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,raosudha89/vowpal_wabbit,tzukuoh/vowpal_wabbit,trufanov-nok/vowpal_wabbit,digitalreasoning/vowpal_wabbit,danmelamed/vowpal_wabbit,zachmayer/vowpal_wabbit,raosudha89/vowpal_wabbit,lhoang29/vowpal_wabbit,lmatthieu/myvw,danmelamed/vowpal_wabbit,zachmayer/vowpal_wabbit,raosudha89/vowpal_wabbit,michaelbrundage/vowpal_wabbit,zachmayer/vowpal_wabbit,pombredanne/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,deaktator/vowpal_wabbit,danmelamed/vowpal_wabbit,marko-asplund/vowpal_wabbit,michaelbrundage/vowpal_wabbit,raosudha89/vowpal_wabbit,danmelamed/vowpal_wabbit,marko-asplund/vowpal_wabbit,marko-asplund/vowpal_wabbit,michaelbrundage/vowpal_wabbit,trufanov-nok/vowpal_wabbit,KaiWeiChang/vowpal_wabbit,pombredanne/vowpal_wabbit,raosudha89/vowpal_wabbit,lhoang29/vowpal_wabbit,lmatthieu/myvw,danmelamed/vowpal_wabbit,digitalreasoning/vowpal_wabbit,martinpopel/vowpal_wabbit,raosudha89/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,trufanov-nok/vowpal_wabbit,trufanov-nok/vowpal_wabbit,lmatthieu/myvw,tzukuoh/vowpal_wabbit,hal3/vowpal_wabbit,tzukuoh/vowpal_wabbit,tzukuoh/vowpal_wabbit,zachmayer/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,martinpopel/vowpal_wabbit,deaktator/vowpal_wabbit,zachmayer/vowpal_wabbit,lmatthieu/myvw,lmatthieu/myvw,martinpopel/vowpal_wabbit,danmelamed/vowpal_wabbit,marko-asplund/vowpal_wabbit,lhoang29/vowpal_wabbit,marko-asplund/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,hal3/vowpal_wabbit,lhoang29/vowpal_wabbit,trufanov-nok/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,pombredanne/vowpal_wabbit,michaelbrundage/vowpal_wabbit,michaelbrundage/vowpal_wabbit