[hack] Fix edge case of empty string in File_content.apply_edit
[hack] Fix edge case of empty string in File_content.apply_edit

Summary: Index out of bounds when editing zero length files.

Reviewed By: andrewjkennedy

Differential Revision: D3929498

fbshipit-source-id: e4cfe35bcb6c37852db012bc02a9b0350727c70c

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