(maint) Fix getparam() spec failure on MRI 1.8

Without this patch applied we're getting the following spec failure, but
only in the MRI 1.8 matrix cells.

    Failures:

      1) getparam when compared against a resource with params
        Failure/Error: should run.with_params('User[dan]', '').and_return('')
        ArgumentError:
          interning empty string

This patch addresses the problem by explicitly returning an empty string if
the string itself is empty.  This avoids trying to convert an empty string to
a symbol which is the root cause of the problem.