zephyr: Handle NULL json results
zephyr: Handle NULL json results

Twister can create "reason" property with NULL value.
This causes an exception when encoding artifact.

BUG=None
BRANCH=None
TEST=./util/zephyr_to_resultdb.py --result=twister-out/twister.json

Signed-off-by: Al Semjonovs <c3a2ecb550cad1e7d5a943ca6241377f7b43ffc4@google.com>
Change-Id: I560ed39bc01c47b5b507e58a03e7a17d3b4a1912
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3782098
Reviewed-by: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@google.com>
Commit-Queue: Yuval Peress <50dc11c009bd4d7e9d9d15a383c1743ce5ee82ef@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec