Handle null and empty string inputs in reverse methods

[wasay10]

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[wasay10]

reverseWithStringConcat:
This check ensures that if the input string is null or empty,
The method returns an empty string instead of proceeding with
the reversal logic, which would lead to exceptions.

reverseWithStringBuilder:
Similar to the previous method, this condition prevents null
or empty string inputs from causing runtime errors and returns
an empty string instead, enhancing the method's reliability.