RFR: 8354791: Use Hashtable.putIfAbsent in CSS constructor

[Andrey Turbanov]

We can use Hashtable.putIfAbsent instead of pair `get`/`put` methods.
It's faster and cleaner.

-------------

Commit messages:
 - [PATCH] CSS. Use Hashtable.putIfAbsent instead of pair get/put

Changes: https://git.openjdk.org/jdk/pull/22032/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22032&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354791
  Stats: 6 lines in 1 file changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22032.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22032/head:pull/22032

PR: https://git.openjdk.org/jdk/pull/22032