[SPARK-8803] handle special characters in elements in crosstab

cc rxin

Having back ticks or null as elements causes problems.
Since elements become column names, we have to drop them from the element as back ticks are special characters.
Having null throws exceptions, we could replace them with empty strings.

Handling back ticks should be improved for 1.5

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #7201 from brkyvz/weird-ct-elements and squashes the following commits:

e06b840 [Burak Yavuz] fix scalastyle
93a0d3f [Burak Yavuz] added tests for NaN and Infinity
9dba6ce [Burak Yavuz] address cr1
db71dbd [Burak Yavuz] handle special characters in elements in crosstab