cmd: fdt: Fix handling of empty properties for fdt get addr and fdt get size

cmd: fdt: Fix handling of empty properties for fdt get addr and fdt get size

It is perfectly valid to request an address or size of FDT property
without value, the only special case if requesting of the value of
FDT property without value. Invert the test such, that properties
without value still set the variable from 'fdt get addr/size' to
address of the property or size of the property, where the later
is 0.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>