Avoid reassigning a parameter value to fix Pyright warnings

[correctmost]

PR Description:

This PR fixes the following Pyright warnings:

archinstall/lib/configuration.py:131:26 - error: Argument of type "Path | None" cannot be assigned to parameter "dest_path" of type "Path" in function "_is_valid_path"
    Type "Path | None" is not assignable to type "Path"
      "None" is not assignable to "Path" (reportArgumentType)
archinstall/lib/configuration.py:132:26 - error: Argument of type "Path | None" cannot be assigned to parameter "dest_path" of type "Path" in function "save_user_config"
    Type "Path | None" is not assignable to type "Path"
      "None" is not assignable to "Path" (reportArgumentType)
archinstall/lib/configuration.py:133:25 - error: Argument of type "Path | None" cannot be assigned to parameter "dest_path" of type "Path" in function "save_user_creds"
    Type "Path | None" is not assignable to type "Path"