Crash on posting metadata with empty value

[sclu1034]

Beancount allows metadata with an empty value.
beancount-import does not.

This leads to a confusing runtime error in compute_metadata_changes, where beancount-import will claim that expected metadata keys couldn't be found, even though both old_meta and new_meta in that function contain those keys.
But since the line with an empty value doesn't match the regex, the loop doesn't get to parse all lines and old_meta_not_seen does not get cleared.