chore(iast): early return if nullptr for type checks (#10799)

Code Security: early return for type checks if comparing `None`

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- 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)

ddtrace/appsec/_iast/_taint_tracking/Utils/StringUtils.cpp

@@ -147,6 +147,9 @@ get_pyobject_size(PyObject* obj)
 bool
 PyIOBase_Check(const PyObject* obj)
 {
+    if (!obj)
+        return false;
+
     try {
         return py::isinstance((PyObject*)obj, safe_import("_io", "_IOBase"));
     } catch (py::error_already_set& err) {
@@ -158,6 +161,9 @@ PyIOBase_Check(const PyObject* obj)
 bool
 PyReMatch_Check(const PyObject* obj)
 {
+    if (!obj)
+        return false;
+
     try {
         return py::isinstance((PyObject*)obj, safe_import("re", "Match"));
     } catch (py::error_already_set& err) {