RakuAST: ignore =finish if there is no code

The =finish construct only makes sense in code.  If it is being used
in a RakuDoc context only, it tried to attach the text to the CompUnit
when there was none, because there was no code.

This now just ignores the =finish text if there is no CompUnit to
attach to.  Potentially, it could also be converted to a RakuDoc,
but this does not actually appear to make sense (yet).

src/Raku/Actions.nqp

@@ -3260,7 +3260,7 @@ class Raku::Actions is HLL::Actions does Raku::CommonActions {
     }
 
     method doc-block:sym<finish>($/) {
-        $*CU.replace-finish-content(~$<finish>);
+        $*CU.replace-finish-content(~$<finish>) if $*CU;
     }
 
     method doc-block:sym<alias>($/) {