…l and LogArg to detect null byrefs (#7986)

## Reason for change
Some methods can legitimately produce null managed byrefs (e.g.,
MemoryMarshal.GetReference(emptySpan)), which makes ref T point to
address 0. Attempting to capture such locals/args would dereference an
invalid reference and could crash in LogLocal/LogArg
(NullReferenceException). This change makes DI runs stable and keeps
probes installed while avoiding unsafe dereferences

## Implementation details
Before reading a ref T capture input, we check Unsafe.IsNullRef(ref
value) and return early to avoid dereferencing an invalid reference.

Current behavior is to skip capturing that member entirely (no
misleading null value for non-nullable types like byte).
Follow-up option: if we decide we want the snapshot to explicitly show
this case, we can reuse the existing snapshot notCapturedReason
mechanism (no new reason added in this PR).

## Test coverage
#7987

## Other details
This PR is part of an effort to make the Snapshot Exploration Test run
successfully end-to-end.
#7988
#7989

## Summary of changes
Add a new test that uncovers two bugs:

1. When a null address is passed into our instrumentation code.
2. When locals or arguments exist but are skipped, resulting in an
invalid JSON structure.

## Implementation details
When a null address is passed to our debugger invoker classes, it must
be handled explicitly by checking `Unsafe.IsNullRef`.

## Other details
The test is currently skipped and will be enabled once the following PRs
are merged:
#7986
#7988
#7989