tools/imximage: set DCD pointer to NULL when its length is 0

When dcd_len is 0 the Write Data command that the set_dcd_rst_v2() routine
generates is empty. This causes HAB to complain that the command is invalid.

--------- HAB Event 1 -----------------
event data:
	0xdb 0x00 0x0c 0x41 0x33 0x06 0xc0 0x00
	0xcc 0x00 0x04 0x04

To fix this set the DCD pointer in the IVT to NULL in this case. The DCD header
itself is still needed for detect_imximage_version() to determine the image
version.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Stefano Babic <sbabic@denx.de>