musicbrainz: Handle missing release date

Some releases in the MusicBrainz database do not have an associated
release date.

src/rupert/main.py

@@ -83,7 +83,7 @@ def format_release(release: Release) -> str:
     return RELEASE_INFO_TMPL.format(
         artist=release.artist_credit_phrase,
         title=release.title,
-        year=release.date.year,
+        year=release.date.year if release.date else 'Unknown Year',
         more_info=', '.join(more_info),
     )
 

src/rupert/musicbrainz.py

@@ -45,7 +45,7 @@ class Release(pydantic.BaseModel):
     artist_credit: List[ArtistCredit] = pydantic.Field(alias='artist-credit')
     artist_credit_phrase: str = pydantic.Field(alias='artist-credit-phrase')
     medium_list: List[Medium] = pydantic.Field(alias='medium-list')
-    date: datetime.date
+    date: Optional[datetime.date] = None
     packaging: Optional[str] = None
     country: Optional[str] = None
     label_info: Optional[List[LabelInfo]] = pydantic.Field(