RFR: 8301964: Expensive fillInStackTrace operation in HttpURLConnection.getLastModified when no last-modified in response

[Andrey Turbanov]

[JDK-8297211](https://bugs.openjdk.org/browse/JDK-8297211) (#11258) missed one case with `last-modified` header. We can avoid expensive NPE.

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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