fix: return non-nullable DIDDocument on Resolver.resolve() (#21)

BREAKING CHANGE: The `Resolver.resolve()` method throws an error (rejects the promise) if the DID document returned by the respective method is `null`.
Also, `null` documents are not cached.

Author: mirceanis

.gitignore

@@ -3,6 +3,7 @@ lib
 .vscode/settings.json
 coverage
 .idea/
+.vscode/
 
 *.log
 

README.md

@@ -110,7 +110,7 @@ export function getResolver() {
     did: string,
     parsed: ParsedDID,
     didResolver: Resolver
-  ): Promise<DIDDocument | null> {
+  ): Promise<DIDDocument> {
     console.log(parsed)
     // {method: 'mymethod', id: 'abcdefg', did: 'did:mymethod:abcdefg/some/path#fragment=123', path: '/some/path', fragment: 'fragment=123'}
     const didDoc = ...// lookup doc