[jdk16] RFR: 8259027: NullPointerException in makeMappedSegment due to NULL Unmapper when length of segment is 0 [v2]

[Maurizio Cimadamore]

> When the size of the memory map is zero, FileChannelImpl returns a `null` Unmapper - this creates issues to the mapped memory segment implementation.
> 
> To fix, I've created an empty mapped segment class which is initialized to sensible defaults, and whose implenentation of force/load etc. do nothing.
> 
> We already had a test for this condition - but the test was missing the `@Test` annotation, so it was not run! I've now beefed up the test a bit to make sure that mapped segment operations do not throw.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Added empty mapping support for different access modes

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk16/pull/83/files
  - new: https://git.openjdk.java.net/jdk16/pull/83/files/10c26170..4dcee6b9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk16&pr=83&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk16&pr=83&range=00-01

  Stats: 23 lines in 2 files changed: 16 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/83.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/83/head:pull/83

PR: https://git.openjdk.java.net/jdk16/pull/83