refactor(yarn): Relax strictness in processAdditionalPackageInfo()

A `package.json` file does not contain a `dist` attribute while the
output of `yarn npm view` does. Otherwise, the two formats are identical
regarding the attributes considered by ORT.

Make the code tolerate absent `dist` attributes to prepare for re-using
it to extract `AdditionalData` from a `package.json`.

Signed-off-by: Frank Viernau <frank_viernau@epam.com>

plugins/package-managers/node/src/main/kotlin/yarn2/Yarn2.kt

@@ -585,11 +585,11 @@ class Yarn2(
         val author = parseNpmAuthors(json)
 
         val dist = json["dist"]
-        var downloadUrl = dist["tarball"].textValueOrEmpty()
+        var downloadUrl = dist?.get("tarball").textValueOrEmpty()
 
         downloadUrl = fixNpmDownloadUrl(downloadUrl)
 
-        val hash = Hash.create(dist["shasum"].textValueOrEmpty())
+        val hash = Hash.create(dist?.get("shasum").textValueOrEmpty())
 
         val vcsFromDownloadUrl = VcsHost.parseUrl(downloadUrl)