fix: treat lone empty StringArray flag as empty slice

Passing --flag="" no longer produces a one-element slice containing
an empty string when the default value is empty.

Fixes #415

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: leno23

string_array.go

@@ -15,7 +15,11 @@ func newStringArrayValue(val []string, p *[]string) *stringArrayValue {
 
 func (s *stringArrayValue) Set(val string) error {
 	if !s.changed {
-		*s.value = []string{val}
+		if val == "" {
+			*s.value = []string{}
+		} else {
+			*s.value = []string{val}
+		}
 		s.changed = true
 	} else {
 		*s.value = append(*s.value, val)

string_array_test.go

@@ -193,6 +193,19 @@ func TestSAWithSpecialChar(t *testing.T) {
 	}
 }
 
+func TestSAEmptyStringFlagValue(t *testing.T) {
+	var sa []string
+	f := setUpSAFlagSet(&sa)
+
+	err := f.Parse([]string{"--sa", ""})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if len(sa) != 0 {
+		t.Fatalf("expected empty array for --sa=\"\", got len=%d %v", len(sa), sa)
+	}
+}
+
 func TestSAAsSliceValue(t *testing.T) {
 	var sa []string
 	f := setUpSAFlagSet(&sa)