Handle empty coordinate maps

[lverns]

Includes the single commit from #29.

Fixes #30.

Given deps.edn as per the bug report:

{:deps {org.clojure/clojure {}}
 :aliases {:newer-stuff
           {:override-deps {org.clojure/clojure {:mvn/version "1.10.0"}}}}}

---

Invoking depot with no args prints:

Checking for old versions in: deps.edn
  All up to date!

---

Invoking with --write

Updating old versions in: deps.edn
  All up to date!

and leaves the file unchanged.

---

Invoking with --every:

Checking for old versions in: deps.edn
  org.clojure/clojure {:mvn/version "1.10.0"} -> {:mvn/version "1.10.1"}

---

Invoking with --every and --write:

Updating old versions in: deps.edn
  org.clojure/clojure {:mvn/version "1.10.0"} -> {:mvn/version "1.10.1"}

and the file is now

{:deps {org.clojure/clojure {}}
 :aliases {:newer-stuff
           {:override-deps {org.clojure/clojure {:mvn/version "1.10.1"}}}}}

[Olical]

Awesome stuff! I'll review and merge at some point soon 😄

[Olical]

🎉

[Olical]

I was thinking this would fix all of Sean's woes but I think this one persists 🤔 I'll have to try it myself #11 (comment)

[Olical]

Thank you very much for the fixes! :D