resolves SCM changes ClientUriBuilder

[ArcturusZhang]

Fixes #4567

Description

We have this if statement in ClientUriBuilder to avoid duplicated separators:

if (value[0] == PathSeparator)
{
    value = value.Substring(1);
}

but in some cases it removes the separator incorrectly when there is no duplicate (as described in the above issue).
This PR refines the check to ensure neither of these happens

Checklist

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