Strange. I just copy-pasted your line into the terminal. No error message. No output at all.
The first line you send found the files with 'track' or 'unknown' in it but returned the wrong lines. This one returns nothing. I don't get any output for FLAC either.

BTW if there is no id3 tag at all it just shows the same as an empty tag. So there isn't need for checking for no output at all. But the need for checking for an empty tag is even more important then for 'track' or 'unknown', because there are more of them.

Code:

$ id3v2 --delete-all 07\ -\ Alex\ Roeka\ -\ Kermis\ in\ ravenstein.mp3 
Stripping id3 tag in "07 - Alex Roeka - Kermis in ravenstein.mp3"...id3v1 and v2 stripped.
$ id3 -Rl 07\ -\ Alex\ Roeka\ -\ Kermis\ in\ ravenstein.mp3 

Filename: 07 - Alex Roeka - Kermis in ravenstein.mp3
Title:                               
Artist:                               
Album:                               
Year:     
Genre: Unknown (255)
Track: 0
Comment:

Actually I just noticed an empty id3 tag returns spaces. Select the text above and you see it. So the check can be on "Title: ". That's "Title:" and two spaces. The first one is there always, the second one means there is nothing in the title tag (or it starts with a space what isn't good too).

But metaflac returns nothing at all if the tag is empty. And yes that is noteworthy. More important than performance. metaflac only return tags when they are filled. So if the word 'TITLE' isn't in the output, the title is empty. If only the artist is filled this is the output. If nothing is filled: no message, just nothing is returned.

Code:

$ metaflac --export-tags-to=- 01.\ The\ Florida\ Airport\ Tape.flac 
ARTIST=Frank Zappa

Is it possible to check on not 'TITLE' or not 'ARTIST' or not 'ALBUM' or not 'TRACKNUMBER'? Or isn't it possible to check that the fast way is nothing is returned?