Make UnknownLicense's fields nullable

[mateuszkwiecinski]

Fixes #73

[github-actions]

Diffuse output:

OLD: diffuse-base-file
NEW: licensee-for-android.jar

       │          compressed          │          uncompressed          
       ├──────────┬──────────┬────────┼───────────┬───────────┬────────
 JAR   │ old      │ new      │ diff   │ old       │ new       │ diff   
───────┼──────────┼──────────┼────────┼───────────┼───────────┼────────
 class │ 67.8 KiB │ 67.9 KiB │ +153 B │   147 KiB │ 147.5 KiB │ +547 B 
 other │  1.6 KiB │  1.6 KiB │    0 B │     189 B │     189 B │    0 B 
───────┼──────────┼──────────┼────────┼───────────┼───────────┼────────
 total │ 69.4 KiB │ 69.5 KiB │ +153 B │ 147.1 KiB │ 147.7 KiB │ +547 B 

 CLASSES │ old │ new │ diff      
─────────┼─────┼─────┼───────────
 classes │  39 │  39 │ 0 (+0 -0) 
 methods │ 357 │ 357 │ 0 (+0 -0) 
  fields │  90 │  90 │ 0 (+0 -0)

JAR

    compressed    │   uncompressed    │                                                                                 
─────────┬────────┼──────────┬────────┤                                                                                 
 size    │ diff   │ size     │ diff   │ path                                                                            
─────────┼────────┼──────────┼────────┼─────────────────────────────────────────────────────────────────────────────────
 2.4 KiB │ +100 B │  5.9 KiB │ +344 B │ ∆ io/github/usefulness/licensee/serialization/UnknownLicenses__serializer.class 
 2.4 KiB │  +53 B │  5.3 KiB │ +203 B │ ∆ io/github/usefulness/licensee/serialization/UnknownLicenses.class             
─────────┼────────┼──────────┼────────┼─────────────────────────────────────────────────────────────────────────────────
 4.8 KiB │ +153 B │ 11.2 KiB │ +547 B │ (total)