RFR: 8305902: (cs) Resolve default Charset only once in StreamEncoder and StreamDecoder [v2]

[Sergey Tsypanov]

> If `charsetName` passed into `forOutputStreamWriter()` method is null then `Charset.forName()` is redundant as we can use already resolved default Charset.

Sergey Tsypanov has updated the pull request incrementally with one additional commit since the last revision:

  Further clean-up

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13443/files
  - new: https://git.openjdk.org/jdk/pull/13443/files/047d5f33..859def4e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13443&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13443&range=00-01

  Stats: 6 lines in 2 files changed: 0 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13443.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13443/head:pull/13443

PR: https://git.openjdk.org/jdk/pull/13443