Fix stale FirmPlayerTest: malformed UUID returns null, not throws

FirmPlayerTest.malformedUuidStillThrows() still asserted the pre-ADT-35 behaviour (that FirmPlayer.getUniqueId() throws IllegalArgumentException on a malformed stored UUID). Commit 28b9763 (ADT-35) made the getter null-safe — it now catches the exception and returns null per the documented @Nullable contract — but left this test asserting the old throwing behaviour. As a result :business:business-api:test fails on develop.

Fix: renamed the test to malformedUuidReturnsNull() and asserted getUniqueId() returns null for a malformed value, matching the documented contract. Removed the now-unused assertThatThrownBy import.

Found while building/installing plugins locally.