LibJS: Verify valid Duraton is made in DifferenceTemporalPlainDate

It shouldn't be possible to create an invalid duration here, so follow
the spec and verify that this succeeds.

Userland/Libraries/LibJS/Runtime/Temporal/PlainDate.cpp

@@ -577,7 +577,7 @@ ThrowCompletionOr<NonnullGCPtr<Duration>> difference_temporal_plain_date(VM& vm,
  }
 
  // 16. Return ! CreateTemporalDuration(sign × result.[[Years]], sign × result.[[Months]], sign × result.[[Weeks]], sign × result.[[Days]], 0, 0, 0, 0, 0, 0).
- return TRY(create_temporal_duration(vm, sign * result->years(), sign * result->months(), sign * result->weeks(), sign * result->days(), 0, 0, 0, 0, 0, 0));
+ return MUST(create_temporal_duration(vm, sign * result->years(), sign * result->months(), sign * result->weeks(), sign * result->days(), 0, 0, 0, 0, 0, 0));
 }
 
 }