Add more specific psalm type for result of StringHelper::base64UrlEncode() method

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[what-the-diff]

PR Summary

• Enhanced type for StringHelper::base64UrlEncode() method results
  This change allows better identification of results returned from the StringHelper::base64UrlEncode() method by assigning a more specific psalm type. This tweak helps in better understandability and handling of the function outputs.

• Introduced an additional test case for base64UrlEncodedStringsProvider()
  An empty string test case was added to the base64UrlEncodedStringsProvider() data provider function in the StringHelperTest class. This inclusion will improve our application's robustness by ensuring the function behaves as expected even when provided with an empty string input.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (2.x@1ffc263). Click here to learn what that means.

Additional details and impacted files

@@          Coverage Diff           @@
##             2.x     #128   +/-   ##
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  Coverage       ?   99.76%           
  Complexity     ?      149           
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  Files          ?        7           
  Lines          ?      428           
  Branches       ?        0           
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  Hits           ?      427           
  Misses         ?        1           
  Partials       ?        0           

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