fix(debugging): handle null literal in expressions [backport 1.17] (#6438)

Backport ddd392e from #6388 to 1.17.

Make sure that we can handle the null literal in the expression
language.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Co-authored-by: Gabriele N. Tornetta <[email protected]>

Author: github-actions[bot]

ddtrace/debugging/_expressions.py

@@ -292,8 +292,8 @@ def _compile_operation(ast):
 
 def _compile_literal(ast):
     # type: (DDASTType) -> Optional[List[Instr]]
-    # literal  =>  <number> | true | false | "string"
-    if not isinstance(ast, (str, int, float, bool)):
+    # literal  =>  <number> | true | false | "string" | null
+    if not (isinstance(ast, (str, int, float, bool)) or ast is None):
         return None
 
     return [Instr("LOAD_CONST", ast)]

releasenotes/notes/fix-debugger-expressions-none-literal-30f3328d2e386f40.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    dynamic instrumentation: handle null literal in conditions and expressions.

tests/debugging/test_expressions.py

@@ -72,6 +72,7 @@ def __getitem__(self, name):
         # Test argument predicates and operations
         ({"contains": [{"ref": "payload"}, "hello"]}, {"payload": "hello world"}, True),
         ({"eq": [{"ref": "hits"}, True]}, {"hits": True}, True),
+        ({"eq": [{"ref": "hits"}, None]}, {"hits": None}, True),
         ({"substring": [{"ref": "payload"}, 4, 7]}, {"payload": "hello world"}, "hello world"[4:7]),
         ({"any": [{"ref": "collection"}, {"isEmpty": {"ref": "@it"}}]}, {"collection": ["foo", "bar", ""]}, True),
         ({"startsWith": [{"ref": "local_string"}, "hello"]}, {"local_string": "hello world!"}, True),