flacToMp3Impl: do not assert fieldName != null, it's handled ok

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
AudioTagCheckerImpl: do not assert file exists, it's handled ok

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
build: enable asserts for junit tests

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>