Use isEmpty() instead of length() > 0 in WordCount Tokenizer

[Diogo-Sardagna]

The change enhances code clarity by using the semantically appropriate method provided by the String API. Although functionally equivalent, ‘isEmpty()’ more clearly communicates the intent of checking for an empty string, aligning with common Java coding practices. This improves maintainability and consistency across the codebase.

[flinkbot]

CI report:

• dc6532de3a44a55b56acf72c0f117b40ae0a2302 Azure: SUCCESS

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