[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.