License files which do not match an SPDX expression are erroneously handled as 'unlicensed'.

[HeyeOpenSource]

What happened:
While working with non-OpenSource Licenses in Golang modules it was discovered, that license files which cannot be matched to an SPDX expression are erroneously handled as 'unlicensed'. (cf. #3090)

This is not the expected behaviour for an existing license file and should hence be considered a bug.

Originally posted by @HeyeOpenSource in #3366 (comment)