StringMapConverter: test empty key.

Author: greenrobot-team

tests/objectbox-java-test/src/test/java/io/objectbox/converter/StringMapConverterTest.java

@@ -21,6 +21,7 @@ public void works() {
         Map<String, String> mapWithValues = new HashMap<>();
         mapWithValues.put("Hello", "Grüezi");
         mapWithValues.put("💡", "Idea");
+        mapWithValues.put("", "Empty String Key");
         convertAndBackThenAssert(mapWithValues);
     }