Drivers: refactor driver_get_variable_property to return an enum.

In order to prepare for introduction of fallbacks, refactor the
function to return its status as an enumeration value. Also, move
the index range check inside the function from python-specific code.

source/blender/python/intern/bpy_rna_driver.cc

@@ -27,45 +27,41 @@ PyObject *pyrna_driver_get_variable_value(const AnimationEvalContext *anim_eval_
                                           DriverVar *dvar,
                                           DriverTarget *dtar)
 {
-  PyObject *driver_arg = nullptr;
   PointerRNA ptr;
   PropertyRNA *prop = nullptr;
   int index;
 
-  if (driver_get_variable_property(anim_eval_context, driver, dvar, dtar, &ptr, &prop, &index)) {
-    if (prop) {
-      if (index != -1) {
-        if (index < RNA_property_array_length(&ptr, prop) && index >= 0) {
-          /* object, property & index */
-          driver_arg = pyrna_array_index(&ptr, prop, index);
-        }
-        else {
-          /* out of range, pass */
-        }
-      }
-      else {
-        /* object & property */
-        const PropertyType type = RNA_property_type(prop);
-        if (type == PROP_ENUM) {
-          /* Note that enum's are converted to strings by default,
-           * we want to avoid that, see: #52213 */
-          driver_arg = PyLong_FromLong(RNA_property_enum_get(&ptr, prop));
-        }
-        else {
-          driver_arg = pyrna_prop_to_py(&ptr, prop);
-        }
-      }
-    }
-    else {
+  switch (driver_get_variable_property(
+      anim_eval_context, driver, dvar, dtar, true, &ptr, &prop, &index))
+  {
+    case DRIVER_VAR_PROPERTY_SUCCESS:
       /* object only */
-      driver_arg = pyrna_struct_CreatePyObject(&ptr);
-    }
-  }
-  else {
-    /* can't resolve path, pass */
+      if (!prop) {
+        return pyrna_struct_CreatePyObject(&ptr);
+      }
+
+      /* object, property & index */
+      if (index >= 0) {
+        return pyrna_array_index(&ptr, prop, index);
+      }
+
+      /* object & property (enum) */
+      if (RNA_property_type(prop) == PROP_ENUM) {
+        /* Note that enum's are converted to strings by default,
+         * we want to avoid that, see: #52213 */
+        return PyLong_FromLong(RNA_property_enum_get(&ptr, prop));
+      }
+
+      /* object & property */
+      return pyrna_prop_to_py(&ptr, prop);
+
+    case DRIVER_VAR_PROPERTY_INVALID:
+    case DRIVER_VAR_PROPERTY_INVALID_INDEX:
+      /* can't resolve path, pass */
+      return nullptr;
   }
 
-  return driver_arg;
+  return nullptr;
 }
 
 PyObject *pyrna_driver_self_from_anim_rna(PathResolvedRNA *anim_rna)