fix https://github.com/sagemathinc/smc/issues/1768 -- "_set_codemirror_to_syncstring → setValueNoJump → diff_main: Error: Null input "
fix https://github.com/sagemathinc/smc/issues/1768 -- "_set_codemirror_to_syncstring → setValueNoJump → diff_main: Error: Null input "

tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc,tscholl2/smc