fix(esp_now): Fix broadcast example and use nullptr

[lucasssvaz]

Description of Change

Fixes broadcast slave example and replaces NULL with nullptr for compiler optimizations.

Tests scenarios

Tested locally

Related links

Closes #11165

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Generated by 🚫 dangerJS against 69c5a64

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Test Results

 76 files   76 suites   12m 45s ⏱️
 38 tests  38 ✅ 0 💤 0 ❌
241 runs  241 ✅ 0 💤 0 ❌

Results for commit 69c5a64.

♻️ This comment has been updated with latest results.

[github-actions]

Memory usage test (comparing PR against master branch)

The table below shows the summary of memory usage change (decrease - increase) in bytes and percentage for each target.

Memory	FLASH [bytes]		FLASH [%]		RAM [bytes]		RAM [%]
Target	DEC	INC	DEC	INC	DEC	INC	DEC	INC
ESP32S3	0	⚠️ +308	0.00	⚠️ +0.04	0	0	0.00	0.00
ESP32S2	0	⚠️ +324	0.00	⚠️ +0.04	0	0	0.00	0.00
ESP32C3	0	⚠️ +226	0.00	⚠️ +0.02	0	0	0.00	0.00
ESP32C6	0	⚠️ +226	0.00	⚠️ +0.02	0	0	0.00	0.00
ESP32	0	⚠️ +292	0.00	⚠️ +0.03	0	0	0.00	0.00

Click to expand the detailed deltas report [usage change in BYTES]

Target	ESP32S3		ESP32S2		ESP32C3		ESP32C6		ESP32
Example	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM	FLASH	RAM
libraries/ESP_NOW/examples/ESP_NOW_Broadcast_Master	0	0	0	0	0	0	0	0	0	0
libraries/ESP_NOW/examples/ESP_NOW_Broadcast_Slave	⚠️ +308	0	⚠️ +324	0	⚠️ +226	0	⚠️ +226	0	⚠️ +292	0
libraries/ESP_NOW/examples/ESP_NOW_Network	0	0	0	0	0	0	0	0	0	0
libraries/ESP_NOW/examples/ESP_NOW_Serial	0	0	0	0	0	0	0	0	0	0

[Copilot]

Pull Request Overview

This PR replaces all usages of NULL with nullptr for improved clarity and potential compiler optimizations, and fixes the ESP-NOW broadcast slave example.

• Update constructors, conditionals, and callback registrations to use nullptr.
• Modify the broadcast slave example to use pointer semantics for masters to avoid dangling pointers on vector reallocation.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
libraries/ESP_NOW/src/ESP32_NOW_Serial.h and ESP32_NOW_Serial.cpp	Replaced NULL checks with nullptr in pointer assignments.
libraries/ESP_NOW/src/ESP32_NOW.h and ESP32_NOW.cpp	Replaced NULL with nullptr for consistency and modern C++ style.
libraries/ESP_NOW/examples/ESP_NOW_Network/ESP_NOW_Network.ino	Updated example to use nullptr; minor comment reordering.
libraries/ESP_NOW/examples/ESP_NOW_Broadcast_Slave/ESP_NOW_Broadcast_Slave.ino	Changed broadcast peer initialization to use pointers.
libraries/ESP_NOW/examples/ESP_NOW_Broadcast_Master/ESP_NOW_Broadcast_Master.ino	Replaced NULL with nullptr; minor code consistency update.