Explicit null checks for avoiding JVM crash.

[desruisseaux]

Reference: https://github.com/bcdev/jpy/pull/126

Changes in this commit:

• Use of Objects.requireNonNull is some places were passing a null argument causes a JVM crash.
• Replace some if (foo == null) throws NullPointerException statements by Objects.requireNonNull.
• Update Javadoc by saying when an argument can be null.
• Opportunistic removal of an unused import statement and trailing spaces.