Allow commit without adding targets

[znewman01]

By making init write an empty targets.json.

The following didn't work before but now does:

tuf init

tuf gen-key root
tuf gen-key targets
tuf gen-key snapshot
tuf gen-key timestamp

tuf sign root.json
# you used to need a `tuf add` here
tuf sign targets.json

tuf snapshot
tuf timestamp

tuf commit

[coveralls]

Pull Request Test Coverage Report for Build 2011993102

• 6 of 8 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.9%) to 74.645%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
repo.go	6	8	75.0%

Totals
Change from base Build 2000571104:	3.9%
Covered Lines:	1260
Relevant Lines:	1688

---

💛 - Coveralls

[asraa]

+1 to this change -- this bothered me!

[joshuagl]

Thanks @znewman01 for this. The change looks good to me, but I'd love to hear from a maintainer who's more actively using go-tuf cc @trishankatdatadog @ethan-lowman-dd @hosseinsia

[trishankatdatadog]

LGTM