fix: PyCapsule_GetDestructor is allowed to return a nullptr destructor

Previously, this code would error out if the destructor happened to be
a nullptr. This is incorrect. nullptrs are allowed for capsule
destructors.

"It is legal for a capsule to have a NULL destructor. This makes a
NULL return code somewhat ambiguous; use PyCapsule_IsValid() or
PyErr_Occurred() to disambiguate."

See:

https://docs.python.org/3/c-api/capsule.html#c.PyCapsule_GetDestructor

I noticed this while working on a type caster related to #3858 DLPack
happens to allow the destructor not to be defined on a capsule, and I
encountered such a case. See:

https://github.com/dmlc/dlpack/blob/e2bdd3bee8cb6501558042633fa59144cc8b7f5f/include/dlpack/dlpack.h#L219

Author: galv

include/pybind11/pytypes.h

@@ -1829,18 +1829,18 @@ class capsule : public object {
             // guard if destructor called while err indicator is set
             error_scope error_guard;
             auto destructor = reinterpret_cast<void (*)(void *)>(PyCapsule_GetContext(o));
-            if (destructor == nullptr) {
-                if (PyErr_Occurred()) {
-                    throw error_already_set();
-                }
-                pybind11_fail("Unable to get capsule context");
+            if (PyErr_Occurred()) {
+                throw error_already_set();
             }
             const char *name = get_name_in_error_scope(o);
             void *ptr = PyCapsule_GetPointer(o, name);
             if (ptr == nullptr) {
                 throw error_already_set();
             }
-            destructor(ptr);
+
+            if (destructor != nullptr) {
+                destructor(ptr);
+            }
         });
 
         if (!m_ptr || PyCapsule_SetContext(m_ptr, (void *) destructor) != 0) {