[SPARK-3944][Core] Using Option[String] where value of String can be null

Author: shitis <ssaxena.ece@gmail.com>

Closes #2795 from Shiti/master and squashes the following commits:

46897d7 [shitis] Using Option Wrapper to convert String with value null to None
SPARK-3803 [MLLIB] ArrayIndexOutOfBoundsException found in executing computePrincipalComponents

Avoid overflow in computing n*(n+1)/2 as much as possible; throw explicit error when Gramian computation will fail due to negative array size; warn about large result when computing Gramian too

Author: Sean Owen <sowen@cloudera.com>

Closes #2801 from srowen/SPARK-3803 and squashes the following commits:

b4e6d92 [Sean Owen] Avoid overflow in computing n*(n+1)/2 as much as possible; throw explicit error when Gramian computation will fail due to negative array size; warn about large result when computing Gramian too