Revert "Cope with NULL query string in ExecInitParallelPlan()."

This reverts commit b3ee4c50.
We don't need it in the wake of the preceding commit, which
added an upstream check that the querystring isn't null.

Discussion: https://postgr.es/m/2197698.1617984583@sss.pgh.pa.us

src/backend/executor/execParallel.c

@@ -647,7 +647,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/* Estimate space for query text. */
-	query_len = estate->es_sourceText ? strlen(estate->es_sourceText) : 0;
+	query_len = strlen(estate->es_sourceText);
 	shm_toc_estimate_chunk(&pcxt->estimator, query_len + 1);
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
@@ -742,10 +742,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate,
 
 	/* Store query string */
 	query_string = shm_toc_allocate(pcxt->toc, query_len + 1);
-	if (query_len == 0)
-		query_string[0] = 0;
-	else
-		memcpy(query_string, estate->es_sourceText, query_len + 1);
+	memcpy(query_string, estate->es_sourceText, query_len + 1);
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_QUERY_TEXT, query_string);
 
 	/* Store serialized PlannedStmt. */