Autocompletion: Try to guess method return type even if subscript base has no value

modules/gdscript/tests/scripts/completion/common/infer_return_type_without_value.cfg

@@ -0,0 +1,5 @@
+[output]
+include=[
+    ; String
+    {"display": "begins_with(…)"},
+]

modules/gdscript/tests/scripts/completion/common/infer_return_type_without_value.gd

@@ -0,0 +1,9 @@
+class B:
+	func to_str(b: int):
+		return str(b)
+
+var a: B
+
+func _ready():
+	a.to_str(10).➡
+    pass