ENH: Test for plain property (empty value).

test/test_caosdb.m

@@ -61,6 +61,9 @@ function test_retrieve_single()
   % Default connection configuration is sufficient.
   c = Caosdb();
 
+  % Plain property must not fail.
+  plain_property = c.retrieve_by_id("101"){1};
+
   % Retrieve a single entity
   violin = c.retrieve_by_id("120"){1};