What would be a code for an action to put something in a tag field if it is empty? Let's say a single dot?
It should only be executed [putting the dot sign] when there is nothing in a given field or if it has only white spaces and nothing more
$regexp(%yourfieldhere%,^$,.)
That's the regexp way.
Give yourself a little task and learn and see if you can manage to do it with the $ifgreater(a,b,x,y) and %len() functions.
There are no empty tag-fields. If they exist, they are not empty. Otherwise they don't exist.
Action "Format value"
TEST <== $if(%TEST%,%TEST%,'.')DD.20170824.1626.CEST
Yes
And there are no passive radiators. Every radiator is a passive cooling element. And yet some are made, marketed and sold as passive ones; on the account that they should not require a fan mounted on them
A figure of speech
Works, thank you
Works, thank you
Although both of those codes the do not take into consideration a case when [unwantedely] only a pause or pauses are present in a given field; but that can be of course managed with a running prior to them a simple action
$trim(%FIELD-NAME-HERE%)
$regexp(%yourfieldhere%,^$,.)
Will be to like to work for fields that may or may not have whiitespaces, would be.
$regexp(%yourfieldhere%,^\s*$,.)I think you meant whitespace/s for pauses... 
No. It is not a figure of speech. There is no such tag in the file and therefore it can't be empty.
The tag-sytems is not a the same as a database with empty entries.
If you have no car you don't say: I have an empty car.
Normal filter-expression in MP3Tag therefore for this case is %tag_example% MISSING.