improve sliceValidateError.Error performance using switch and strings.Builder (#2765)

fix missing nil pointer check

use simpler switch case

add missing tests

use for-loop instead of range

add benchmark test codes

binding/default_validator_test.go

@@ -16,6 +16,26 @@ func TestSliceValidateError(t *testing.T) {
 		want string
 	}{
 		{"has nil elements", sliceValidateError{errors.New("test error"), nil}, "[0]: test error"},
+		{"has zero elements", sliceValidateError{}, ""},
+		{"has one element", sliceValidateError{errors.New("test one error")}, "[0]: test one error"},
+		{"has two elements",
+			sliceValidateError{
+				errors.New("first error"),
+				errors.New("second error"),
+			},
+			"[0]: first error\n[1]: second error",
+		},
+		{"has many elements",
+			sliceValidateError{
+				errors.New("first error"),
+				errors.New("second error"),
+				nil,
+				nil,
+				nil,
+				errors.New("last error"),
+			},
+			"[0]: first error\n[1]: second error\n[5]: last error",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {