SPARK-3178  setting SPARK_WORKER_MEMORY to a value without a label (m or g) sets the worker memory limit to zero

Validate the memory is greater than zero when set from the SPARK_WORKER_MEMORY environment variable or command line without a g or m label.  Added unit tests. If memory is 0 an IllegalStateException is thrown. Updated unit tests to mock environment variables by subclassing SparkConf (tip provided by Josh Rosen).   Updated WorkerArguments to use SparkConf.getenv instead of System.getenv for reading the SPARK_WORKER_MEMORY environment variable.

Author: Bill Bejeck <bbejeck@gmail.com>

Closes #2309 from bbejeck/spark-memory-worker and squashes the following commits:

51cf915 [Bill Bejeck] SPARK-3178 - Validate the memory is greater than zero when set from the SPARK_WORKER_MEMORY environment variable or command line without a g or m label.  Added unit tests. If memory is 0 an IllegalStateException is thrown. Updated unit tests to mock environment variables by subclassing SparkConf (tip provided by Josh Rosen).   Updated WorkerArguments to use SparkConf.getenv instead of System.getenv for reading the SPARK_WORKER_MEMORY environment variable.
[SPARK-3946] gitignore in /python includes wrong directory

Modified to ignore not the docs/ directory, but only the docs/_build/ which is the output directory of sphinx build.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2796 from tsudukim/feature/SPARK-3946 and squashes the following commits:

2bea6a9 [Masayoshi TSUZUKI] [SPARK-3946] gitignore in /python includes wrong directory
[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
[SPARK-3869] ./bin/spark-class miss Java version with _JAVA_OPTIONS set

When _JAVA_OPTIONS environment variable is set, a command "java -version" outputs a message like "Picked up _JAVA_OPTIONS: -Dfile.encoding=UTF-8".
./bin/spark-class knows java version from the first line of "java -version" output, so it mistakes java version with _JAVA_OPTIONS set.

Author: cocoatomo <cocoatomo77@gmail.com>

Closes #2725 from cocoatomo/issues/3869-mistake-java-version and squashes the following commits:

f894ebd [cocoatomo] [SPARK-3869] ./bin/spark-class miss Java version with _JAVA_OPTIONS set
[SPARK-3943] Some scripts bin\*.cmd pollutes environment variables in Windows

Modified not to pollute environment variables.
Just moved the main logic into `XXX2.cmd` from `XXX.cmd`, and call `XXX2.cmd` with cmd command in `XXX.cmd`.
`pyspark.cmd` and `spark-class.cmd` are already using the same way, but `spark-shell.cmd`, `spark-submit.cmd` and `/python/docs/make.bat` are not.

Author: Masayoshi TSUZUKI <tsudukim@oss.nttdata.co.jp>

Closes #2797 from tsudukim/feature/SPARK-3943 and squashes the following commits:

b397a7d [Masayoshi TSUZUKI] [SPARK-3943] Some scripts bin\*.cmd pollutes environment variables in Windows