explain: Fix displaying Delta joins (#35422)

Broken in #32262

Author: def-

src/compute-types/src/plan/render_plan.rs

@@ -888,8 +888,8 @@ impl<'a, T> std::fmt::Display for RenderPlanExprHumanizer<'a, T> {
                     for dpp in path_plans {
                         if !first {
                             write!(f, " ")?;
-                            first = false;
                         }
+                        first = false;
                         write!(f, "[%{}", dpp.source_relation)?;
 
                         for dsp in &dpp.stage_plans {

test/testdrive/explain-analyze.td

@@ -93,7 +93,7 @@ operator                     total_elapsed
 "              Read l2" <XXX>
 "  Arrange (#0, #3)" <XXX>
 "    Stream l2" <null>
-"l2 = Delta Join [%0 » %1 » %2 » %3 » %4 » %5][%1 » %0 » %2 » %3 » %4 » %5][%2 » %0 » %1 » %3 » %4 » %5][%3 » %0 » %1 » %2 » %4 » %5][%4 » %0 » %1 » %2 » %3 » %5][%5 » %0 » %1 » %2 » %3 » %4]" <XXX>
+"l2 = Delta Join [%0 » %1 » %2 » %3 » %4 » %5] [%1 » %0 » %2 » %3 » %4 » %5] [%2 » %0 » %1 » %3 » %4 » %5] [%3 » %0 » %1 » %2 » %4 » %5] [%4 » %0 » %1 » %2 » %3 » %5] [%5 » %0 » %1 » %2 » %3 » %4]" <XXX>
 "  Arrange (#0{replica_id})" <XXX>
 "    Arranged l0" <null>
 "  Arrange (#0{replica_id}, #3{bucket_start})" <XXX>
@@ -121,7 +121,7 @@ operator                     total_elapsed
 "    Map/Filter/Project" <XXX>
 "      Accumulable GroupAggregate" <XXX>
 "        Map/Filter/Project" <XXX>
-"          Delta Join [%0 » %2 » %1][%1 » %0 » %2][%2 » %0 » %1]" <XXX>
+"          Delta Join [%0 » %2 » %1] [%1 » %0 » %2] [%2 » %0 » %1]" <XXX>
 "            Arranged <XXX>" <null>
 "            Arranged <XXX>" <null>
 "            Arrange (#0{replica_id}) (#1{size})" <XXX>