Minor: compute qualify window expressions only when QUALIFY clause is present (#21173)

## Which issue does this PR close?
Does not close but related #608


## Rationale for this change
Followup to #20963
@jonahgao had a feedback in
#20963 (comment)
to try to find qualify only if we are inside QUALIFY block. I did not
think it was this minor so thought followup would be better but this is
simply moving to if block to prevent unnecessary computation


## What changes are included in this PR?
Move `qualify_window_func_exprs` computation inside the QUALIFY block

## Are these changes tested?
Covered by existing tests from #20963.

## Are there any user-facing changes?
No

Co-authored-by: Jonah Gao <jonahgao@msn.com>

Author: buraksenn

datafusion/sql/src/select.rs

@@ -293,14 +293,6 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
             plan
         };
 
-        // The window expressions from SELECT and QUALIFY only, used to validate that
-        // QUALIFY is used with window functions (ORDER BY window functions don't count).
-        let qualify_window_func_exprs = find_window_exprs(
-            select_exprs_post_aggr
-                .iter()
-                .chain(qualify_expr_post_aggr.iter()),
-        );
-
         // All of the window expressions (deduplicated and rewritten to reference aggregates as
         // columns from input). Window functions may be sourced from the SELECT list, QUALIFY
         // expression, or ORDER BY.
@@ -342,6 +334,11 @@ impl<S: ContextProvider> SqlToRel<'_, S> {
         // QUALIFY filters the results of window functions, similar to how HAVING filters aggregates
         let plan = if let Some(qualify_expr) = qualify_expr_post_aggr {
             // Validate that QUALIFY is used with window functions in SELECT or QUALIFY
+            let qualify_window_func_exprs = find_window_exprs(
+                select_exprs_post_aggr
+                    .iter()
+                    .chain(std::iter::once(&qualify_expr)),
+            );
             if qualify_window_func_exprs.is_empty() {
                 return plan_err!(
                     "QUALIFY clause requires window functions in the SELECT list or QUALIFY clause"