[FIX] base: keep first multiline translation in po file

The first term of a po file is a comment for translator e.g.:
msgid ""
msgstr ""
"Project-Id-Version: openobject-addons\n"
...

This comment is ignored if there is no source and it is the first term of the po
file. The first flage was disabled too late and if the following terms also
started with an empty source (for too long terms), they were skipped as well.

Disable the flag as soon as the condition is evaluated to make sure no
additional terms are ignored. opw 619786