[SPARK-3375] spark on yarn container allocation issues

If yarn doesn't get the containers immediately it stops asking for them and the yarn application hangs with never getting any executors.

The issue here is that we are sending the number of containers as 0 after we send the original one of X. on the yarn side this clears out the original request.

For a ping we should just send empty asks.

Author: Thomas Graves <tgraves@apache.org>

Closes #2275 from tgravescs/SPARK-3375 and squashes the following commits:

74b6820 [Thomas Graves] send empty resource requests when we aren't asking for containers

yarn/alpha/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocationHandler.scala

@@ -51,12 +51,13 @@ private[yarn] class YarnAllocationHandler(
   override protected def allocateContainers(count: Int): YarnAllocateResponse = {
     var resourceRequests: List[ResourceRequest] = null
 
-    // default.
-    if (count <= 0 || preferredHostToCount.isEmpty) {
-      logDebug("numExecutors: " + count + ", host preferences: " +
-        preferredHostToCount.isEmpty)
-      resourceRequests = List(createResourceRequest(
-        AllocationType.ANY, null, count, YarnSparkHadoopUtil.RM_REQUEST_PRIORITY))
+    logDebug("numExecutors: " + count)
+    if (count <= 0) {
+      resourceRequests = List()
+    } else if (preferredHostToCount.isEmpty) {
+        logDebug("host preferences is empty")
+        resourceRequests = List(createResourceRequest(
+          AllocationType.ANY, null, count, YarnSparkHadoopUtil.RM_REQUEST_PRIORITY))
     } else {
       // request for all hosts in preferred nodes and for numExecutors -
       // candidates.size, request by default allocation policy.

yarn/stable/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocationHandler.scala

@@ -88,9 +88,11 @@ private[yarn] class YarnAllocationHandler(
 
   private def addResourceRequests(numExecutors: Int) {
     val containerRequests: List[ContainerRequest] =
-      if (numExecutors <= 0 || preferredHostToCount.isEmpty) {
-        logDebug("numExecutors: " + numExecutors + ", host preferences: " +
-          preferredHostToCount.isEmpty)
+      if (numExecutors <= 0) {
+        logDebug("numExecutors: " + numExecutors)
+        List()
+      } else if (preferredHostToCount.isEmpty) {
+        logDebug("host preferences is empty")
         createResourceRequests(
           AllocationType.ANY,
           resource = null,