Saving empty target segments to TM is currently not possible. While a completed file may be downloaded with some empty translations, the corresponding empty TM entries cannot be saved. 

Current behavior:

Segments with empty target or a target containing a whitespace character (space, non-breaking space, zero-width, full-width, and thin space) are not saved to TM. 

Workaround:

Enter the unicode character "U+2800" (Braille Pattern Blank). However, in some systems, it can appear garbled.

Expected behavior:

TM should allow a more "common" space character to be saved.

Use case:
To prevent the same segment appearing untranslated each time when a variant of the initial source file is pre-translated.