This is a simple cleanup to replace the sentinel `HtmlTree.EMPTY` text constant with an instance that achieves the same by overriding `isValid()`. I think this is the nicer solution, and it allows us to remove the special case identity check in `HtmlTree.add(Content)`.
------------- Commit messages: - JDK-8267394: Do not rely on object identity for empty valid Content instance Changes: https://git.openjdk.java.net/jdk/pull/4130/files Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4130&range=00 Issue: https://bugs.openjdk.java.net/browse/JDK-8267394 Stats: 37 lines in 8 files changed: 19 ins; 9 del; 9 mod Patch: https://git.openjdk.java.net/jdk/pull/4130.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/4130/head:pull/4130 PR: https://git.openjdk.java.net/jdk/pull/4130
