R1:d280b24239fc

Fix "arc paste" to stop creating pastes with an empty string ("") as the "language"

Summary:
See PHI652. When you `echo x | arc paste` today, you end up with a Paste object that has the empty string as its "language".

This is normally not valid. Pastes where the language should be autodetected should have the value `null`, not the empty string.

This behavior likely changed when `paste.create` got rewritten in terms of `paste.edit`. Adjust the implementation so it only adds the LANGUAGE transaction if…