[jdk17u-dev] RFR: 8272702: Resolving URI relative path with no / may lead to incorrect toString

[Radim Vansa]

I backport this as it affects to JDK 17.
Does not apply clean due to a conflict with https://bugs.openjdk.org/browse/JDK-8285521 but it's the same logic.

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### Progress
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### Error
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### Reviewing
<details><summary>Using <code>git</code></summary>

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`$ git checkout pull/1176`

Update a local copy of the PR: \
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Commit messages:
 - Backport 79597f1ea6844f374beeeba219719cd9d5fe7d03

Changes: https://git.openjdk.org/jdk17u-dev/pull/1176/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1176&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8272702
  Stats: 29 lines in 2 files changed: 24 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1176.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/1176/head:pull/1176

PR: https://git.openjdk.org/jdk17u-dev/pull/1176