Observed that SequencedMap:firstEntry() throws NullPointerEx when the map entry has null key or null value.

Example : the following code snippet throws NPEx.
======== ( When key is null )
SequencedMap<String, String> sequencedMap = new LinkedHashMap<>();
        sequencedMap.put(null, "v1");
        sequencedMap.firstEntry();

======== (When value is null )
SequencedMap<String, String> sequencedMap = new LinkedHashMap<>();
        sequencedMap.put("k", null);
        sequencedMap.firstEntry();

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