Missing Null pointer checks in CommandLine.java

• CommandLine.java #L233
  Method call to `getValuesList()` returns the possible values of `Option` for which it is called as List or returns `null`. In case it returns `null`, `addAll()` method call on type `List` with `null` as an argument will lead to a NullPointerException. This can be prevented by explicitly checking and handling the cases when it might return null. I have made changes for the same

• CommandLine.java #L260
  As per the implementation of `stripLeadingHyphens(String)`, there exists cases when it might return `null`. And calling `equals` method on a null return value (#L263 ) will lead to null pointer exception. So as to prevent this I have made changes to explicitly return null in case `stripLeadingHyphens` returns null

• CommandLine.java #L472  
  There is a possibility that `addOption` method is called with null as an argument. In that case we will be adding null values to our list of processed options (as `List` does not thrown any null pointer exception even if `add` method is called with null values). So as to prevent this we need to make sure that value that we are adding to list of processed option is not null.

I have created a pull request for addressing these issues, see