Fix function example in expression language component

The is_string check needs to be part of the runtime code, not of the compilation code, because it needs to check that the argument is a string, not the compiled code to access it (which is always a string as it is source code
Removed return statement
bug #3771 Fix function example in expression language component (raulfraile)

This PR was submitted for the master branch but it was merged into the 2.4 branch instead (closes #3771).

Discussion
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Fix function example in expression language component

The is_string check needs to be part of the runtime code, not of the compilation code, because it needs to check that the argument is a string, not the compiled code to access it (which is always a string as it is source code. cc @stof

Commits
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db710c4 Removed return statement
bc673f3 Fix function example in expression language component