ZEPPELIN-1901. Output schema might be null for PigQueryInterpreter

[zjffdu]

What is this PR for?

The output schema might be null if user doesn't specify it explicitly. In this PR, I will use 'col_{pos}' to replace the column name if it is null.

What type of PR is it?

[Improvement]

Todos

• - Task

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-1901

How should this be tested?

Unit test is added.

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[zjffdu]

@felixcheung Please help review, I will add document in #1830 to encourage user to specify schema explicitly.

[zjffdu]

Ping @felixcheung , CI failure is not relevant.

AngularElem
- should provide onclick method *** FAILED ***
  The code passed to eventually never returned normally. Attempted 1 times over 192.484891 milliseconds. Last failure message: 0 was not equal to 1. (AbstractAngularElemTest.scala:64)

[felixcheung]

LGTM
merging if no more comment