ai.nwchem: no stray ##DispersionCorrection## line in DFT decks

DispersionCorrection returned an empty string without setting $_, so the
template engine left the tag line in every DFT deck with dispersion None
(the default). NWChem skipped it as a comment. Found by tests/teaching.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0167GfjZWhsGJh9at1F9sc9y

Author: andy

scripts/parsers/ai.nwchem

@@ -1952,6 +1952,9 @@ sub DispersionCorrection {
       && $dispersionModel{$choice} ne "") {
     $result = "disp " . $dispersionModel{$choice};
   }
+  # The engine keeps the template line unless $_ is set, as the other
+  # resolvers do; without it "##DispersionCorrection##" stayed in the deck.
+  $_ = $result;
   return $result;
 }