How do I find blank entries and replace with a character

How do I find blank entries in a field (say 'Album Artists') then replace with a character (say a '.') I want to force a field structure for all music files
I sometimes edit tags for one album from the right context menu >Edit Tag (dBpoweramp) but I don't want to have to type in field names each time and possibly mistype them.

To filter for files with missing fields use (e.g.)
%albumartist% MISSING
as filter

To set a value to a field, either use the tag panel, the function Convert>Tag-Tag or an action of the type "Format value" for the field or the extended tags dialogue.
And just a footnote: a field without contents is simply not there. There is no reserved place in the tag for a specific field.