Reject on empty fields object in PlainYearMonth::with (#626)

Small fix for a bug in `PlainYearMonth::with`.

This can still be handled downstream in the engines, if there is careful
attention to the specification steps. We should still reject here to fix
bug in Rust API.

Author: nekevss

src/builtins/core/plain_year_month.rs

@@ -542,6 +542,11 @@ impl PlainYearMonth {
         // 4. Let calendar be yearMonth.[[Calendar]].
         // 5. Let fields be ISODateToFields(calendar, yearMonth.[[ISODate]], year-month).
         // 6. Let partialYearMonth be ? PrepareCalendarFields(calendar, temporalYearMonthLike, « year, month, month-code », « », partial).
+        if fields.is_empty() {
+            return Err(
+                TemporalError::r#type().with_message("plainYearMonth fields cannot be empty")
+            );
+        }
         // 7. Set fields to CalendarMergeFields(calendar, fields, partialYearMonth).
         // 8. Let resolvedOptions be ? GetOptionsObject(options).
         // 9. Let overflow be ? GetTemporalOverflowOption(resolvedOptions).
@@ -942,19 +947,17 @@ mod tests {
         ); // assert month code has changed
         assert_eq!(with_month_code.iso_month(), 5); // month is changed as well
 
-        // Day
-        let fields = YearMonthCalendarFields::new();
-        let with_day = base.with(fields, None).unwrap();
-        assert_eq!(with_day.iso_year(), 2025); // year is not changed
-        assert_eq!(with_day.iso_month(), 3); // month is not changed
-        assert_eq!(with_day.iso.day, 1); // day is ignored
-
         // All
         let fields = YearMonthCalendarFields::new().with_year(2001).with_month(2);
         let with_all = base.with(fields, None).unwrap();
         assert_eq!(with_all.iso_year(), 2001); // year is changed
         assert_eq!(with_all.iso_month(), 2); // month is changed
         assert_eq!(with_all.iso.day, 1); // day is ignored
+
+        // Empty fields -> throws error
+        let fields = YearMonthCalendarFields::new();
+        let empty = base.with(fields, None);
+        assert!(empty.is_err());
     }
 
     #[test]