inprocess: return correct eof markers from StrFile:read.

* specs/inprocess_spec.yaml (with read): Specify that read returns
the empty string for end of file when called with "*a", or nil
otherwise.
* lib/specl/inprocess (StrFile:read): Handle "*a" early, so that
the empty string is returned, otherwise return nil for end of file
before further processing.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>