Use C++ 'nullptr' in place of 'NULL'

[thinkyhead]

It is now recommended to use nullptr which is the C++ language-supported equivalent to the NULL macro. Other than being a language built-in and not a macro, there are some minor advantages to this small modernization, such as not needing to cast nullptr to the current pointer type.

[ManuelMcLure]

Looks like it already found somewhere NULL was being used incorrectly.

[ManuelMcLure]

My search for places where there's a (<type>*)0 shows the following locations:

• src/HAL/HAL_AVR/endstop_interrupts.h lines 57 & 67
• src/HAL/HAL_STM32F4/HAL_spi_STM32F4.cpp line 168

No other occurrences that I can find with regular expression \*\s*)\s*0 other than some false positives where there is a cast from a hex constant to a pointer type.

[InsanityAutomation]

As a side note, the null pointers in host_actions caused pio to fail compiling todays build which is what prompted me to look at this. I replaced null with nullptr and all is well, just on the 2 lines in the header.