RFR: 8270490: Charset.forName() taking fallback default value [v2]

[Naoto Sato]

> During the review of JEP 400, a proposal to provide an overloaded method to `Charset.forName()` was suggested [[1]](https://github.com/openjdk/jdk/pull/4733#discussion_r669693954). This PR is to implement the proposal. A CSR is also drafted as https://bugs.openjdk.java.net/browse/JDK-8275348

Naoto Sato has updated the pull request incrementally with one additional commit since the last revision:

  Moved the null sentence into @param tag.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6045/files
  - new: https://git.openjdk.java.net/jdk/pull/6045/files/a31dbdc7..4c2142be

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6045&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6045&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6045.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6045/head:pull/6045

PR: https://git.openjdk.java.net/jdk/pull/6045