Steps to replicate the issue (include links if applicable):

• Go to https://translatewiki.net/wiki/Special:Translate?filter=&action=page&language=ppl&group=ext-0-wikimedia&uselang=en
• Type null in the "Filter list" box.

What happens?:

Many untranslated messages are shown as results, even though the source string doesn't have the string "null".

What should have happened instead?:

Untranslated messages must be shown only if the source message has the string "null".

The empty content is probably treated as the string "null" somewhere.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

What is currently deployed on translatewiki.