drivers: flash: esp32: handle zero-length reads as no-op

Add a guard clause in flash_esp32_read() to return 0 when the
requested length is zero. This avoids unnecessary operations
and aligns with expected flash API behavior, where reading
zero bytes is treated as a no-op.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>