Prefer `0` over `NULL` as function pointers

SunC warns use of `NULL`, pointer to data as function pointers.

class.c

@@ -249,7 +249,7 @@ class_alloc(VALUE flags, VALUE klass)
      */
     RCLASS_SET_ORIGIN((VALUE)obj, (VALUE)obj);
     RB_OBJ_WRITE(obj, &RCLASS_REFINED_CLASS(obj), Qnil);
-    RCLASS_SET_ALLOCATOR((VALUE)obj, NULL);
+    RCLASS_SET_ALLOCATOR((VALUE)obj, 0);
 
     return (VALUE)obj;
 }

internal/class.h

@@ -148,7 +148,7 @@ static inline rb_alloc_func_t
 RCLASS_ALLOCATOR(VALUE klass)
 {
     if (FL_TEST_RAW(klass, FL_SINGLETON)) {
-        return NULL;
+        return 0;
     }
     return RCLASS_EXT(klass)->as.class.allocator;
 }

ractor.c

@@ -648,7 +648,7 @@ ractor_sleep_with_cleanup(rb_execution_context_t *ec, rb_ractor_t *cr, enum rb_r
 static enum rb_ractor_wakeup_status
 ractor_sleep(rb_execution_context_t *ec, rb_ractor_t *cr, enum rb_ractor_wait_status wait_status)
 {
-    return ractor_sleep_with_cleanup(ec, cr, wait_status, NULL, NULL);
+    return ractor_sleep_with_cleanup(ec, cr, wait_status, 0, NULL);
 }
 
 // Ractor.receive

struct.c

@@ -1871,7 +1871,7 @@ rb_data_with(int argc, const VALUE *argv, VALUE self)
     }
 
     VALUE h = rb_struct_to_h(self);
-    rb_hash_update_by(h, kwargs, NULL);
+    rb_hash_update_by(h, kwargs, 0);
     return rb_class_new_instance_kw(1, &h, rb_obj_class(self), TRUE);
 }
 

time.c

@@ -5828,7 +5828,7 @@ Init_Time(void)
 
     if (debug_find_time_numguess) {
         rb_define_hooked_variable("$find_time_numguess", (VALUE *)&find_time_numguess,
-                                  find_time_numguess_getter, NULL);
+                                  find_time_numguess_getter, 0);
     }
 
     rb_cTimeTM = Init_tm(rb_cTime, "tm");