Mono/C#: Fix Variant -> MonoString* when type is Variant:NIL

`Variant::operator String()` returns "Null" if the type is `Variant:NIL`.
We must consider that and return a null `MonoString*` instead when marshalling.
This was also causing a "Null" error to be displayed when exporting a game
because null string members would be set to "Null" during hot-reload.

2 files changed, 10 insertions, 2 deletions

diff --git a/modules/mono/mono_gd/gd_mono_field.cpp b/modules/mono/mono_gd/gd_mono_field.cpp
index d84359b1ab..56a9bf46a3 100644
--- a/modules/mono/mono_gd/gd_mono_field.cpp
+++ b/modules/mono/mono_gd/gd_mono_field.cpp
@@ -110,8 +110,14 @@ void GDMonoField::set_value_from_variant(MonoObject *p_object, const Variant &p_
 		} break;
 
 		case MONO_TYPE_STRING: {
-			MonoString *mono_string = GDMonoMarshal::mono_string_from_godot(p_value);
-			mono_field_set_value(p_object, mono_field, mono_string);
+			if (p_value.get_type() == Variant::NIL) {
+				// Otherwise, Variant -> String would return the string "Null"
+				MonoString *mono_string = NULL;
+				mono_field_set_value(p_object, mono_field, mono_string);
+			} else {
+				MonoString *mono_string = GDMonoMarshal::mono_string_from_godot(p_value);
+				mono_field_set_value(p_object, mono_field, mono_string);
+			}
 		} break;
 
 		case MONO_TYPE_VALUETYPE: {
diff --git a/modules/mono/mono_gd/gd_mono_marshal.cpp b/modules/mono/mono_gd/gd_mono_marshal.cpp
index f74fe5715c..9f55b31edc 100644
--- a/modules/mono/mono_gd/gd_mono_marshal.cpp
+++ b/modules/mono/mono_gd/gd_mono_marshal.cpp
@@ -374,6 +374,8 @@ MonoObject *variant_to_mono_object(const Variant *p_var, const ManagedType &p_ty
 		}
 
 		case MONO_TYPE_STRING: {
+			if (p_var->get_type() == Variant::NIL)
+				return NULL; // Otherwise, Variant -> String would return the string "Null"
 			return (MonoObject *)mono_string_from_godot(p_var->operator String());
 		} break;