Resolve clang-tidy error:

```
include/pybind11/detail/type_caster_base.h:795:21: error: the 'empty' method should be used to check for emptiness instead of 'size' [readability-container-size-empty,-warnings-as-errors]
                if (matching_bases.size() != 0) {
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~
                    !matching_bases.empty()
```

include/pybind11/detail/type_caster_base.h

@@ -792,7 +792,7 @@ class type_caster_generic {
                         matching_bases.push_back(base);
                     }
                 }
-                if (matching_bases.size() != 0) {
+                if (!matching_bases.empty()) {
                     if (matching_bases.size() > 1) {
                         matching_bases.push_back(const_cast<type_info *>(typeinfo));
                         all_type_info_check_for_divergence(matching_bases);