[SPARK-18382][WEBUI] "run at null:-1" in UI when no file/line info in call site info

## What changes were proposed in this pull request?

Avoid reporting null/-1 file / line number in call sites if encountering StackTraceElement without this info

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #15862 from srowen/SPARK-18382.

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -1419,8 +1419,12 @@ private[spark] object Utils extends Logging {
             }
             callStack(0) = ste.toString // Put last Spark method on top of the stack trace.
           } else {
-            firstUserLine = ste.getLineNumber
-            firstUserFile = ste.getFileName
+            if (ste.getFileName != null) {
+              firstUserFile = ste.getFileName
+              if (ste.getLineNumber >= 0) {
+                firstUserLine = ste.getLineNumber
+              }
+            }
             callStack += ste.toString
             insideSpark = false
           }