[go-server] Add ability to handle nullable query param

[lwj5]

Add the ability to handle nullable query param

nullable query params will now be passed as a pointer to the servicer.
non-nullable query params will still be passed as value.

There is no breaking changes in this PR

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[lwj5]

test cases added

cc @icubbon for any other comments

[wing328]

lgtm but I've not tested it locally.