Fix crash on get index for non-existing string

As _buttons and _axes have both valid string and nullptr.
When iterating over them, if given key exists it will work correctly.
But if given key does not exist, it will end up with
String::operator=(nullptr). As String constructor from nullptr exists, I
use it.

main/input_default.cpp

@@ -1338,11 +1338,14 @@ String InputDefault::get_joy_button_string(int p_button) {
 
 int InputDefault::get_joy_button_index_from_string(String p_button) {
 	for (int i = 0; i < JOY_BUTTON_MAX; i++) {
-		if (p_button == _buttons[i]) {
+		if (_buttons[i] == nullptr) {
+			break;
+		}
+		if (p_button == String(_buttons[i])) {
 			return i;
 		}
 	}
-	ERR_FAIL_V(-1);
+	ERR_FAIL_V_MSG(-1, vformat("Could not find a button index matching the string \"%s\".", p_button));
 }
 
 int InputDefault::get_unused_joy_id() {
@@ -1361,9 +1364,12 @@ String InputDefault::get_joy_axis_string(int p_axis) {
 
 int InputDefault::get_joy_axis_index_from_string(String p_axis) {
 	for (int i = 0; i < JOY_AXIS_MAX; i++) {
-		if (p_axis == _axes[i]) {
+		if (_axes[i] == nullptr) {
+			break;
+		}
+		if (p_axis == String(_axes[i])) {
 			return i;
 		}
 	}
-	ERR_FAIL_V(-1);
+	ERR_FAIL_V_MSG(-1, vformat("Could not find an axis index matching the string \"%s\".", p_axis));
 }