permit SPIFFS_write() on NULL buffer

[benpicco]

SPIFFS_write(&spi_fs, fd, buf, count) with buf = NULL can be valid when dumping the firmware of a microcontroller with internal flash mapped to address 0x0.

SPIFFS_write(&spi_fs, fd, 0x0, firmware_size) would previously omit the first page when dumping the firmware to flash (all 0xFF).

Visualization of the problem:

Use len instead to signal where only page header is stored in spiffs_page_allocate_data.

[christianeisendle]

My 2 cents on this: If NULL points on a specific platform to a valid address, then the defintion for NULL should be changed instead removing NULL pointer checks from code.

[benpicco]

Well it's not really a NULL pointer check in this case rather than the use of the NULL parameter for in-band signalling. It's just as valid to use the len parameter for in-band signaling here without breaking this use-case.