RFR: 8367049: URL.openConnection throws StringIndexOutOfBoundsException in avm mode

[Oumaiyma Intissar]

Constructing URLPermission with an empty/missing host in the authority (e.g., `"http:///path"`) could throw `StringIndexOutOfBoundsException`.

**Problem**
Empty or malformed authorities reach HostPortrange, which does `charAt(0)` without checking, causing `StringIndexOutOfBoundsException`.

**Fix**
- `URLPermission.Authority`: after stripping userinfo, fail fast if host part is empty.
- `HostPortrange`: add guards for null/empty input and leading ':' (port without host).
- No `HttpURLConnection` changes needed in JDK 26 (the `SecurityManager` permission path is gone).

**Compatibility**
Only affects malformed inputs: previously `StringIndexOutOfBoundsException`, now `IllegalArgumentException`. Valid inputs unaffected.

**Testing**
New jtreg test: `test/jdk/java/net/URLPermission/EmptyAuthorityTest.java` verifies `IllegalArgumentException` for malformed authorities and success for valid ones.

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Commit messages:
 - 8367049: URL.openConnection throws StringIndexOutOfBoundsException in avm mode

Changes: https://git.openjdk.org/jdk/pull/27896/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27896&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367049
  Stats: 81 lines in 3 files changed: 81 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27896.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27896/head:pull/27896

PR: https://git.openjdk.org/jdk/pull/27896