Fix RNA path resolving failing to lookup empty string properties

An empty string is a valid ID property name, when set, RNA's
internal lookup function (rna_path_token_in_brackets), considered
the path invalid.

Now quoted tokens are allowed to be empty.

This fixes Python exceptions being thrown in the custom property editor
with empty custom property names.

source/blender/makesrna/intern/rna_path.cc

@@ -125,9 +125,12 @@ static char *rna_path_token_in_brackets(const char **path,
     return nullptr;
   }
 
-  /* Empty, return. */
-  if (UNLIKELY(len == 0)) {
-    return nullptr;
+  /* Support empty strings in quotes, as this is a valid key for an ID-property. */
+  if (!quoted) {
+    /* Empty, return. */
+    if (UNLIKELY(len == 0)) {
+      return nullptr;
+    }
   }
 
   /* Try to use fixed buffer if possible. */

tests/python/bl_pyapi_idprop.py

@@ -281,6 +281,8 @@ class TestRNAData(TestHelper, unittest.TestCase):
             '[""]',
             '["""]',
             '[""""]',
+            # Empty properties are also valid.
+            "",
         )
         for key_id in keys_to_test:
             self.id[key_id] = 1