Ensure empty previous values for msgid and msgstr are properly saved.

[samhocevar]

This patch prevents polib from creating illegal pofiles (that have #| msgctxt but not #| msgid).

Here is a simple way to witness the behaviour:

before.po

msgid ""
msgstr ""

#, fuzzy
#| msgctxt "oldcontext"
#| msgid ""
msgctxt "newcontext"
msgid ""
msgstr "After"

test.py

import polib
polib.pofile('before.po').save('after.po')

Result

By running test.py you can observe that the #| msgid "" line has disappeared. This makes the .po file invalid, and tools such as e.g. msgmerge will no longer be able to open it. For instance:

# cp after.po after.pot
# msgmerge after.po after.pot
after.po:7:8: syntax error
msgmerge: found 1 fatal error
#

[izimobil]

Merged, thanks !