RFR: 8261290: Improve error message for NumberFormatException on null input

[Joe Darcy]

In Integer and Long, several of the parsing methods are specified to throw NumberFormatException on a null input rather than an NPE. That behavior is not proposed to be modified. However, I think it is reasonable for the NumberFormatException to have a more informative error message than just "null" -- specifically "Cannot parse null string" is proposed here.

All four prior occurrences of 

throw new NumberFormatException("null")

have be updated; there were no other instances of this idiom in the java.lang package.

All java/lang/{Integer, Long} regression tests pass with this change.

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Commit messages:
 - Initial commit.

Changes: https://git.openjdk.java.net/jdk/pull/2663/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2663&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261290
  Stats: 6 lines in 2 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2663.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2663/head:pull/2663

PR: https://git.openjdk.java.net/jdk/pull/2663