This was introduced in #26481 and supposed to fixed in #27250. However the fix was not complete. It leads KeyStore uninitialized if no ssl certificate present

The lesson learned

• even trivial change (eliminate nullness warning to make checkStyle happy) can introduce bug
• Add testing is encouraged. in this case, a smoke test would have revealed the problem missed in Handling issue where keystore file is empty for MongoDB SSL #27250

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