Prior to this commit, ThreeStepCrypticClue simply returned a
concatenation of the active step locations without filtering the mapped
locations in any way. This could lead to NPEs in the plugin as some
cryptic clues have null locations for steps which have no specific
location or have a variable location. This commit addresses this by
making CrypticClue's location field `@Nullable`, filtering null
locations from ThreeStepCrypticClue's getLocations stream, and by adding
a test to ensure ThreeStepCrypticClue's getLocations method cannot yield
any null entries in its return value.
70dc7f56bc