[code-reflection] RFR: Report no attribute present when extracting attribute value from externalized op

[Paul Sandoz]

`ExternalizedOp.extractAttributeValue` is changed to report `null` to the mapping function if no attribute is present in the attribute map (a `null` value is represented by the value `ExternalizedOp.NULL_ATTRIBUTE_VALUE`) . This enables the caller to support a default value when no attribute is present.

Variable operations (`VarOp`) now correctly support variables with no names when instantiating from an externalized op.

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Commit messages:
 - Doc tweak.
 - Support processing externalized var ops with no name.

Changes: https://git.openjdk.org/babylon/pull/150/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=150&range=00
  Stats: 59 lines in 7 files changed: 30 ins; 2 del; 27 mod
  Patch: https://git.openjdk.org/babylon/pull/150.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/150/head:pull/150

PR: https://git.openjdk.org/babylon/pull/150