[JDK-8297211](https://bugs.openjdk.org/browse/JDK-8297211) (#11258) missed one case with `last-modified` header. We can avoid expensive NPE.
------------- Commit messages: - [PATCH] Expensive fillInStackTrace operation in HttpURLConnection.getLastModified when no last-modified in response Changes: https://git.openjdk.org/jdk/pull/12451/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12451&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8301964 Stats: 9 lines in 1 file changed: 4 ins; 2 del; 3 mod Patch: https://git.openjdk.org/jdk/pull/12451.diff Fetch: git fetch https://git.openjdk.org/jdk pull/12451/head:pull/12451 PR: https://git.openjdk.org/jdk/pull/12451
