NIFI-15461: Fixed bug that caused system tests to fail (#48)

Author: markap14

src/main/java/org/apache/nifi/components/connector/AbstractConnector.java

@@ -204,7 +204,7 @@ public CompletableFuture<Void> drainFlowFiles(final FlowContext flowContext) {
             return null;
         });
 
-        startNonSourceFuture.thenRun(() -> {
+        startNonSourceFuture.thenRunAsync(() -> {
             try {
                 ensureDrainageUnblocked();
             } catch (final Exception e) {