[SPARK-16138] Try to cancel executor requests only if we have at least 1

## What changes were proposed in this pull request?
Adding additional check to if statement

## How was this patch tested?
I built and deployed to internal cluster to observe behaviour. After the change the invalid logging is gone:

```
16/06/22 08:46:36 INFO yarn.YarnAllocator: Driver requested a total number of 1 executor(s).
16/06/22 08:46:36 INFO yarn.YarnAllocator: Canceling requests for 1 executor container(s) to have a new desired total 1 executors.
16/06/22 08:46:36 INFO yarn.YarnAllocator: Driver requested a total number of 0 executor(s).
16/06/22 08:47:36 INFO yarn.ApplicationMaster$AMEndpoint: Driver requested to kill executor(s) 1.
```

Author: Peter Ableda <abledapeter@gmail.com>

Closes #13850 from peterableda/patch-2.

yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala

@@ -352,7 +352,7 @@ private[yarn] class YarnAllocator(
         logInfo(s"Submitted container request (host: ${hostStr(request)}, capability: $resource)")
       }
 
-    } else if (missing < 0) {
+    } else if (numPendingAllocate > 0 && missing < 0) {
       val numToCancel = math.min(numPendingAllocate, -missing)
       logInfo(s"Canceling requests for $numToCancel executor container(s) to have a new desired " +
         s"total $targetNumExecutors executors.")