[Java] Fix key lookup returning null clashing with default value (#7236)

A field with key attribute must always be written on the message so it
can be looked up by key. There is a edge case where inserting a key
field with same value as default would prevent it to be written on
the message and later cannot be found when searched by key.

tests/dictionary_lookup.fbs

@@ -0,0 +1,11 @@
+namespace DictionaryLookup;
+
+table LongFloatEntry {
+  key: long (key);
+  value: float;
+}
+
+table LongFloatMap {
+  entries: [LongFloatEntry];
+}
+root_type LongFloatMap;