Fix invalid elm code for paths containing UUIDs

[mawis]

Fixes #12504

This fixes a problem in the template, that generates the function used to serialize the values of elm types to a string, that can be used within a query path.

For strings the function used for conversion is identity and for UUIDs then have to be converted to a string using Uuid.toString. The problem with the existing template is, that it adds both functions to the generated code. In case that we have a UUID we should only convert it to a string using Uuid.toString. The call to identity is superfluous and syntactically incorrect as well.

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@eriktim

[eriktim]

Thanks!