BUG: Fix unsupplied DType path in dispatching

This did not actually trigger any problems, probably because the
reduce fallback mode later just catches it anyway...

On the upside, this probably fixes most of the caching issues ;)

Author: seberg

numpy/core/src/umath/dispatching.c

@@ -254,10 +254,6 @@ resolve_implementation_info(PyUFuncObject *ufunc,
             for (Py_ssize_t i = 0; i < nargs; i++) {
                 int best;
 
-                /* Whether this (normally output) dtype was specified at all */
-                npy_bool is_not_specified = (
-                        op_dtypes[i] == (PyArray_DTypeMeta *)Py_None);
-
                 PyObject *prev_dtype = PyTuple_GET_ITEM(best_dtypes, i);
                 PyObject *new_dtype = PyTuple_GET_ITEM(curr_dtypes, i);
 
@@ -271,7 +267,8 @@ resolve_implementation_info(PyUFuncObject *ufunc,
                  *       the subclass should be considered a better match
                  *       (subclasses are always more specific).
                  */
-                if (is_not_specified) {
+                /* Whether this (normally output) dtype was specified at all */
+                if (op_dtypes[i] == NULL) {
                     /*
                      * When DType is completely unspecified, prefer abstract
                      * over concrete, assuming it will resolve.