RFR: 8301964: Expensive fillInStackTrace operation in HttpURLConnection.getLastModified when no last-modified in response [v2]

[Andrey Turbanov]

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

Andrey Turbanov has updated the pull request incrementally with one additional commit since the last revision:

  8301964: Expensive fillInStackTrace operation in HttpURLConnection.getLastModified when no last-modified in response
  
  rename Default to defaultValue

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12451/files
  - new: https://git.openjdk.org/jdk/pull/12451/files/a585c3c9..b836a2c3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12451&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12451&range=00-01

  Stats: 23 lines in 3 files changed: 0 ins; 0 del; 23 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