[SPARK-5771][UI][hotfix] Change Requested Cores into * if default cores is not set

[jerryshao]

cc @andrewor14, @srowen.

[srowen]

Yes that looks good.

[SparkQA]

Test build #28032 has started for PR 4800 at commit a2483c2.

• This patch merges cleanly.

[andrewor14]

Yes, LGTM thanks for fixing this. I'll merge it into master once tests pass.

[SparkQA]

Test build #28032 has finished for PR 4800 at commit a2483c2.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28032/
Test FAILed.

[andrewor14]

retest this please

[SparkQA]

Test build #28041 has started for PR 4800 at commit a2483c2.

• This patch merges cleanly.

[SparkQA]

Test build #28041 has finished for PR 4800 at commit a2483c2.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/28041/
Test PASSed.

[andrewor14]

Per discussion on #4841 I'm reverting this.