fix: Runtime error when Pandas Series is not always of string type

[fanszoro]

Pull Request Description

I. Change Description

This commit fixes the runtime error that occurs when the series is not always of the string type. In the previous code logic, there might have been an overly simplistic assumption about the type of series. When the series contains non - string type data, it leads to runtime errors, affecting the normal operation of the program. This fix adds type checking and corresponding handling logic to ensure that the program can run correctly regardless of the type of series, enhancing the robustness and stability of the code.

[mergify]

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[nikos-livathinos]

@fanszoro your code needs a proper formatting.
Please check here how to install and run pre-commit.

[nikos-livathinos]

Looks good to me!