RFR: 8270903: sun.net.httpserver.HttpConnection: Improve toString

[Julia Boes]

This is a minor change that updates `sun.net.httpserver.HttpConnection::toString` to never return null.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.java.net/jdk/pull/4928/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4928&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270903
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4928.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4928/head:pull/4928

PR: https://git.openjdk.java.net/jdk/pull/4928