Add logic to handle uuid in Elm generator (0.19)

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

To fix the following issue reported in the chat room by @mxinden

Exception in thread "main" java.lang.RuntimeException: Could not generate api file for 'Silence'
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:596)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:828)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:342)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:62)
Caused by: java.lang.RuntimeException: Parameter 'silenceID' cannot be converted to a string
        at org.openapitools.codegen.languages.ElmClientCodegen.paramToString(ElmClientCodegen.java:542)
        at org.openapitools.codegen.languages.ElmClientCodegen.postProcessOperationsWithModels(ElmClientCodegen.java:446)
        at org.openapitools.codegen.DefaultGenerator.processOperations(DefaultGenerator.java:1037)
        at org.openapitools.codegen.DefaultGenerator.generateApis(DefaultGenerator.java:507)
        ... 3 more

Spec: https://github.com/prometheus/alertmanager/blob/master/api/v2/openapi.yaml

cc @trenneman