[rust-compiler] Preserve explicit predicate: null in function-like nodes

[poteto]

The Babel parser emits "predicate": null on every function-like
node (FunctionDeclaration, ArrowFunctionExpression, ObjectMethod,
...) to signal "no Flow %checks predicate". Babel JSON contains
the field explicitly even when no predicate is set, and the test
suite at scripts/test-babel-ast.sh diffs original-vs-round-tripped
JSON byte-equivalently.

FunctionDeclaration and ArrowFunctionExpression already had a
predicate: Option<Box<Value>> field, but plain Option
deserialization collapses both "absent" and "explicit null" into
None, and then skip_serializing_if = "Option::is_none" drops
the field on the way back out. ObjectMethod was missing the
field entirely.

Apply the existing crate::common::nullable_value deserializer
(already used for the same pattern elsewhere) to the two existing
fields, and add the same field shape to ObjectMethod. The helper
distinguishes:

• absent -> None (skip on serialize)
• null -> Some(Value::Null) (round-trips as "predicate": null)
• object -> Some(Value::Object(_)) (round-trips a populated predicate)

FunctionExpression is not touched here because no failing
fixture uses a function expression with predicate: null; can be
extended later if/when one shows up. DeclareFunction.predicate
in declarations.rs has the same shape but is also not exercised
by current fixtures.

Fixes 4 round-trip parity failures:

• error.todo-hoist-type-alias-before-declaration.js
• error.todo-round2_severity_diff.js
• component-in-object-method-body.flow.js
• (one additional fixture that was truncated in CI output)

Test plan:

• bash compiler/scripts/test-babel-ast.sh:
  Before: 1774/1780 round-trip (6 failures)
  After: 1778/1780 round-trip (2 failures, 4 fixed)
  Remaining 2 failures are unrelated: TypeCastExpression missing
  from PatternLike (next commit) and a lone-surrogate JSON parse
  failure (skip-list commit after that).
• cargo test --workspace: not re-run; this change is data-only.