refactor: simplify presence-check doc and reuse the helper

Drop the explicit intra-doc link reference on `has_matchable_build_rows`: the
inline `[`NullEquality::NullEqualsNothing`]` link resolves on its own since the
type is in scope, so spelling out the path again is redundant.

Also use `has_matchable_build_rows()` for the empty-build-side check in
`HashJoinStream` instead of a raw `map().is_empty()`, keeping the presence
checks consistent. No behavior change.

Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>

Author: Phoenix500526

datafusion/physical-plan/src/joins/hash_join/exec.rs

@@ -242,8 +242,6 @@ impl JoinLeftData {
     /// Under [`NullEquality::NullEqualsNothing`] build rows whose join key is
     /// NULL are omitted from the map, so this can be `false` even when
     /// [`Self::has_build_rows`] is `true`.
-    ///
-    /// [`NullEquality::NullEqualsNothing`]: datafusion_common::NullEquality::NullEqualsNothing
     pub(super) fn has_matchable_build_rows(&self) -> bool {
         !self.map().is_empty()
     }

datafusion/physical-plan/src/joins/hash_join/stream.rs

@@ -779,7 +779,7 @@ impl HashJoinStream {
             }
         }
 
-        let is_empty = build_side.left_data.map().is_empty();
+        let is_empty = !build_side.left_data.has_matchable_build_rows();
 
         if is_empty {
             let result = build_batch_empty_build_side(