[common] Fix candidate filtering.

The filtering was done on the thruthiness of the symbol
string content.
4 jobs for fix_no_change_filtering in 4 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #195134
docker-build
build_linux_36

00:02:54

55.0%
passed #195135
docker-build
build_linux_37

00:02:47

55.0%
passed #195132
macosx
build_macosx_36

00:04:00

55.0%
passed #195133
macosx
build_macosx_37

00:03:47

55.0%