fix(streams): tighten stream typings and guard optional runtime paths for duplex and wrapper utilities

ts/smartstream.classes.streamwrapper.ts

@@ -59,7 +59,7 @@ export class StreamWrapper {
     }
 
     // combine the stream
-    let finalStream = null;
+    let finalStream: plugins.stream.Duplex | null = null;
     let firstIteration: boolean = true;
     for (const stream of streamExecutionArray) {
       if (firstIteration === true) {
@@ -71,12 +71,17 @@ export class StreamWrapper {
       for (const customEventObject of this.customEventObjectArray) {
         stream.on(customEventObject.eventName, customEventObject.eventFunction);
       }
-      if (!firstIteration) {
+      if (!firstIteration && finalStream) {
         finalStream = finalStream.pipe(stream);
       }
       firstIteration = false;
     }
 
+    if (!finalStream) {
+      done.resolve();
+      return done.promise;
+    }
+
     this.streamStartedDeferred.resolve();
 
     let resolved = false;