fix(validatefilter): drop the dead nobs assignment, keep targetds precedence

Review feedback on the merged fix:

- derived_nobs was assigned twice, the first one dead - the coalesced form below
  is the live value.  Removed, so there is one assignment to read.
- making the catalog test a plain %if after the derived-variables test let the
  catalog branch override it; the derived view won before and still should.  The
  catalog branch now requires derived_nobs=0, so the view keeps precedence and
  only a catalog with no derived configuration is blanked.

Two guarded if blocks, not %else %if: that construct is what the parser rejected
in this file before ('Expected DO not found'), and the first attempt at this
review point reproduced it - validatefilter and viewdata both failed to complete
again until it was replaced.

Verified: sasjs test -t new -> 64/64 (100%), TEST=0.

sas/sasjs/services/public/validatefilter.sas

@@ -78,8 +78,6 @@ run;
   outview=work.vw_derived,
   outvars=work.derived_vars
 )
-%let derived_nobs=%mf_nobs(work.derived_vars);
-
 /* A format catalog (LIBREF.CATALOGNAME-FC) must be left for mp_filterstore to
   rewrite into a cntlout dataset, and a -FC name handed over as targetds is not
   something it can read - so pass no targetds at all in that case.  An empty
@@ -96,7 +94,10 @@ run;
 %if &derived_nobs>0 %then %do;
   %let targetds_val=work.vw_derived;
 %end;
-%if &is_fmtcat=1 %then %do;
+/* the derived view keeps precedence: a catalog is only blanked when there is no
+  derived configuration to validate against.  Two plain %if blocks rather than
+  %else %if - that construct is what the parser rejected here before. */
+%if &derived_nobs=0 and &is_fmtcat=1 %then %do;
   %let targetds_val=;
 %end;