py: mp_buffer_info_t::buf may be valid, but NULL for empty objects.

This happens for example for zero-size arrays. As .get_buffer() method now
has explicit return value, it's enough to distinguish success vs failure
of getting buffer.

py/obj.c

@@ -410,7 +410,7 @@ bool mp_get_buffer(mp_obj_t obj, mp_buffer_info_t *bufinfo, int flags) {
         return false;
     }
     int ret = type->buffer_p.get_buffer(obj, bufinfo, flags);
-    if (ret != 0 || bufinfo->buf == NULL) {
+    if (ret != 0) {
         return false;
     }
     return true;

py/obj.h

@@ -210,7 +210,7 @@ typedef struct _mp_buffer_info_t {
     // them with ver = sizeof(struct). Cons: overkill for *micro*?
     //int ver; // ?
 
-    void *buf;
+    void *buf;    // can be NULL if len == 0
     mp_int_t len; // in bytes
     int typecode; // as per binary.h