Issue title: Compilation failure with :node-library single exports
 Issue body: So I have a `shadow-cljs.edn` like this:

```
{:source-paths ["src"]
 :dependencies []
 :builds {:prod {:output-to "dist/ep-specs.js"
                 :output-dir "dist"
                 :optimizations :whitespace
                 :source-map "dist/ep-specs.js.map"
                 :source-map-timestamp false
                 :target :node-library
                 :verbose true
                 :compiler-stats true
                 :exports ep.spec/-main}}}
```

And when I compile with `shadow-cljs compile :prod` I get:

```
[:prod] Compiling ...
IllegalArgumentException: Don't know how to create ISeq from: clojure.lang.Symbol
	clojure.lang.RT.seqFrom (RT.java:550)
	clojure.lang.RT.seq (RT.java:530)
	clojure.core/seq--5124 (core.clj:137)
	clojure.core/seq--5124 (core.clj:137)
	shadow.build.targets.node-library/check-exports! (node_library.clj:113)
	shadow.build.targets.node-library/check-exports! (node_library.clj:109)
	shadow.build.targets.node-library/process (node_library.clj:158)
	shadow.build.targets.node-library/process (node_library.clj:151)
```

Using `:exports {:dummy ep.spec/-main}` fixes it:

```
shadow-cljs - starting ...
[:prod] Compiling ...
[:prod] Build completed. (25 files, 1 compiled, 0 warnings, 2.07s)
Done in 25.46s.
```
 Comments: 
 Comment 0: Fixed in `2.1.23`.

Just FYI: Most of the config you provided does not have any effect since thats now how shadow-cljs operates. ;)

```
{:source-paths ["src"]
 :dependencies []
 :builds {:prod {:output-to "dist/ep-specs.js"
                 :target :node-library
                 :exports ep.spec/-main}}}
```

You can specify `:output-dir` but it is not required.
 Comment 1: `:prod` is also not the ideal name for a build since `shadow-cljs` has dev/release mode built-in

```
{:source-paths ["src"]
 :dependencies []
 :builds {:lib {:output-to "dist/ep-specs.js"
                :target :node-library
                :exports ep.spec/-main}}}
```

`shadow-cljs watch lib` or `shadow-cljs compile lib` during development. `shadow-cljs release lib` for optimized release build. Same config.
 Comment 2: Ok cool will fix that
{'source': 'GithubIssues', 'repo_name_with_owner': 'thheller/shadow-cljs', 'issue_id': 299178544, 'repo_stars': 1950.0}