[foreign-memaccess+abi] RFR: Remove specialized zero-lenght mapped segment

[Per Minborg]

This PR proposes to remove the specialized zero-length MappedMemorySegment.

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Commit messages:
 - Remove specialized zero-lenght mapped segment

Changes: https://git.openjdk.org/panama-foreign/pull/793/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=793&range=00
  Stats: 46 lines in 4 files changed: 13 ins; 25 del; 8 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/793.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/793/head:pull/793

PR: https://git.openjdk.org/panama-foreign/pull/793