fix: close issue 59446 make oct("") same as mysql (#61767)

close pingcap/tidb#59446

pkg/expression/builtin_string.go

@@ -2993,6 +2993,11 @@ func (b *builtinOctStringSig) evalString(ctx EvalContext, row chunk.Row) (string
 		return "", isNull, err
 	}
 
+	// for issue #59446 should return NULL for empty string
+	if len(val) == 0 {
+		return "", true, nil
+	}
+
 	negative, overflow := false, false
 	val = getValidPrefix(strings.TrimSpace(val), 10)
 	if len(val) == 0 {

pkg/expression/builtin_string_vec.go

@@ -1774,8 +1774,15 @@ func (b *builtinOctStringSig) vecEvalString(ctx EvalContext, input *chunk.Chunk,
 			result.AppendNull()
 			continue
 		}
-		negative, overflow := false, false
+
+		// for issue #59446 should return NULL for empty string
 		str := buf.GetString(i)
+		if len(str) == 0 {
+			result.AppendNull()
+			continue
+		}
+
+		negative, overflow := false, false
 		str = getValidPrefix(strings.TrimSpace(str), 10)
 		if len(str) == 0 {
 			result.AppendString("0")