I want to batch replace empty (null) comments fields with a text string describing my library .But if after selecting my files, in replace i leave the original field blank or use empty quotes ("") and put my string in replace id does not work. Is there a specific character/string to represent empty fields.
A Null-field ist not there, so there is nowhere to replace something in.
Instead you could simply select all the tracks with empty comments, enter the comment string in the tag panel and press Ctrl-S or the disk symbol in the tool bar.
Or you could write an action of the type "Format tag field".
In Mp3tag this character seqence ... "" ... is not an empty string, but a string of two quote characters.
In Mp3tag a string is delimited by a pair of single apostroph characters, e. g. ...
'This is a string.'
In MP3tag you cannot create an empty string by writing two apostrophes side by side ... '' ... because the apostrophe is a special character and two apostrophes side by side will result into one apostroph.