issue #892

[BenYang12]

Fixes Or Enhances for issue #892

When a custom type converter returns nil, validation currently fails before a callback registered with RegisterValidationCtx(..., true) can even inspect the context or accept the value. My change lets nil-enabled callbacks run on the resulting reflect.Invalid value, including values returned by ValidatorValuer and plain nil inputs.

The validation loop checks the nil opt-in for each subsequent validator. Validators without that opt-in still fail for invalid values without being called. Error construction avoids reflection panics and retains nil type/value metadata.

Regression tests cover callback invocation and context propagation, acceptance/rejection, typed and converted nil inputs, pointer converters, AND/OR chains, aliases, omission tags, conditional built-ins, and error metadata.

• Tests exist or have been written that cover this particular change.

Validation

Verified commit 369ef5447f99afb020bfa84dbf6907940846fd30:

• go test -count=1 -race -covermode=atomic ./... passes; main package coverage is 96.9%.
• golangci-lint run passes with the CI-pinned v2.13.1: 0 issues.
• git diff --check upstream/master...HEAD passes; both changed Go files are gofmt-clean.
• The new regression tests fail against the unchanged upstream validator.go at dfe35cf8317892133dfe31e36054dcbf36aab604, confirming the missing callback behavior.

Unresolved v10 compatibility question

I made this intentionally a draft pending maintainer guidance. The change also allows already nil-enabled built-ins (required_*, excluded_*, and skip_unless) to run on invalid values. As a result, some converted-nil inputs that previously failed unconditionally now pass when the condition permits absence. Custom nil-enabled callbacks can now receive an invalid reflect.Value and must handle it safely.

@go-playground/validator-maintainers