Guard against nullptr+0 UB in call() and fix assert bound

- Add early return when size_==0 to avoid nullptr+0 pointer
  arithmetic (UB per C++ standard, even though all ESPHome
  targets treat it as no-op)
- Fix debug assert: size < UINT16_MAX allows capacity up to 65535

esphome/core/helpers.cpp

@@ -24,7 +24,7 @@ static const char *const TAG = "helpers";
 
 __attribute__((noinline, cold)) void *callback_manager_grow(void *data, uint16_t size, uint16_t &capacity,
                                                             size_t elem_size) {
-  ESPHOME_DEBUG_ASSERT(size < UINT16_MAX - 1);
+  ESPHOME_DEBUG_ASSERT(size < UINT16_MAX);
   uint16_t new_cap = size + 1;
   auto *new_data = ::operator new(new_cap *elem_size);
   if (data) {

esphome/core/helpers.h

@@ -1834,6 +1834,9 @@ template<typename... Ts> class CallbackManager<void(Ts...)> {
 
   /// Call all callbacks in this manager.
   inline void ESPHOME_ALWAYS_INLINE call(Ts... args) {
+    if (this->size_ == 0) {
+      return;
+    }
     for (auto *it = this->data_, *end = it + this->size_; it != end; ++it) {
       it->call(args...);
     }