Fix 500 error when namespace path is a negative integer

What does this MR do and why?

• Contributes to #339606 (closed)
• Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3333735

Problem

Negative integer is provided to PG lower function that does not support integers.

PG::UndefinedFunction: ERROR:  function lower(integer) does not exist

Solution

• Adjust Regex to handle negative integers
• Explicitly convert path to string type

How to set up and validate locally

#339606 (closed)

1. Create a project fork with Rest API

curl -X POST -H "PRIVATE-TOKEN: _TOKEN_" "http://127.0.0.1:3000/api/v4/projects/1/fork?namespace_id=-1"

2. You should not see a 500 error

MR acceptance checklist

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