Allow empty strings in set string conversions

enginetest/queries/script_queries.go

@@ -9764,6 +9764,8 @@ where
 				},
 			},
 			{
+				// this is failing due to a type coercion bug in comparison.Compare
+				// https://github.com/dolthub/dolt/issues/9510
 				Skip:  true,
 				Query: "select i, s + 0, s from t where s = '';",
 				Expected: []sql.Row{
@@ -9773,7 +9775,6 @@ where
 				},
 			},
 			{
-				Skip:  true,
 				Query: "select i, s + 0, s from tt;",
 				Expected: []sql.Row{
 					{0, float64(3), "something,"},

sql/types/set.go

@@ -347,9 +347,6 @@ func (t SetType) convertBitFieldToString(bitField uint64) (string, error) {
 			if !ok {
 				return "", sql.ErrInvalidSetValue.New(bitField)
 			}
-			if len(val) == 0 {
-				continue
-			}
 			if writeCommas {
 				strBuilder.WriteByte(',')
 			} else {
@@ -367,6 +364,7 @@ func (t SetType) convertStringToBitField(str string) (uint64, error) {
 		return 0, nil
 	}
 	var bitField uint64
+	_, allowEmptyString := t.valToBit[""]
 	lastI := 0
 	var val string
 	for i := 0; i < len(str)+1; i++ {
@@ -375,7 +373,7 @@ func (t SetType) convertStringToBitField(str string) (uint64, error) {
 		}
 
 		// empty string should hash to 0, so just skip
-		if lastI == i {
+		if lastI == i && !allowEmptyString {
 			lastI = i + 1
 			continue
 		}

sql/types/set_test.go

@@ -272,7 +272,7 @@ func TestSetConvertToString(t *testing.T) {
 		bit         uint64
 		expectedStr string
 	}{
-		{[]string{"", "a", "b", "c"}, sql.Collation_Default, 15, "a,b,c"},
+		{[]string{"", "a", "b", "c"}, sql.Collation_Default, 15, ",a,b,c"},
 		{[]string{"", "a", "b", "c"}, sql.Collation_Default, 14, "a,b,c"},
 	}