[FIX] Small safety/cleanup in Nitrite interface (map lookups, closed check, name trim)

[AKakkar28]

Make some cleanups to:

• Avoid double map lookups & NPE risk in the keyed-repository checks
• Centralize closed-state logic in checkOpened()
• Trim collection names in validateCollectionName

@anidotnet let me know if I can cleanup the Nitrite.java and contribute to this project. Thanks a lot.

[anidotnet]

Yes, why not. You are absolutely welcome to make contribution.