fixes SI-7636 NPE in typer

Not sure what lead to fn.tpe being null and if avoiding this "case" is the right solution, but it fixes the issue and all tests pass, except files/run/macro-typemacros-fifth-proto which did not pass before the change. 


**files/run/macro-typemacros-fifth-proto-run.log:**
<pre>
Impls_Macros_2.scala:71: error: `=', `&gt;:', or `&lt;:' expected
  type H2Db(url: String) = macro impl
           ^
one error found
</pre>