fixes an issue where get_block would raise exception
fixes an issue where get_block would raise exception

The get_block method will raise an exception if it cannot find the requested
parent string.  Now the get_block method will return none in this case.

devtodor/pyeapi-py3,SivagnanamCiena/pyeapi,mitchmahan/pyeapi,mzbenami/pyeapi,jerearista/pyeapi,arista-eosplus/pyeapi