[foreign-memaccess+abi] RFR: Change local variable from Optional to String

[Per Minborg]

This PR suggests removing the storage of `Optional` instance variables and instead adhere to the recommendations of Optional usage. This will also reduce memory usage slightly.

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Commit messages:
 - Change local variable from Optional to String

Changes: https://git.openjdk.org/panama-foreign/pull/907/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=907&range=00
  Stats: 63 lines in 7 files changed: 5 ins; 2 del; 56 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/907.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/907/head:pull/907

PR: https://git.openjdk.org/panama-foreign/pull/907