fix(fossid-webapp): Make Snippet url property non-nullable

This is a follow-up of the discussion at [1]: Since FossID does not
document the data model of the API response, the `url` property of the
`Snippet` has been defensively made nullable.
Since then, FossID support confirmed that the property is never `null`
in the API responses: While the database schema indeed has this property
as nullable, the CLI tool used by the scanner guarantee it is never
null.

[1]: oss-review-toolkit#6791 (comment)

Signed-off-by: Nicolas Nobelis <nicolas.nobelis@bosch.io>

clients/fossid-webapp/src/main/kotlin/model/result/Snippet.kt

@@ -38,7 +38,7 @@ data class Snippet(
 
     val file: String,
     val fileLicense: String?,
-    val url: String?,
+    val url: String,
     val hits: String,
     val size: Int?,
 

scanner/src/main/kotlin/scanners/fossid/FossId.kt

@@ -822,10 +822,8 @@ class FossId internal constructor(
 
                 // FossID does not return the hash of the remote artifact. Instead, it returns the MD5 hash of the
                 // matched file in the remote artifact as part of the "match_file_id" property.
-                val snippetProvenance = it.url?.let { url ->
-                    ArtifactProvenance(RemoteArtifact(url, Hash.NONE))
-                } ?: UnknownProvenance
-                val purlType = it.url?.let { url -> urlToPackageType(url).toString() }
+                val snippetProvenance = ArtifactProvenance(RemoteArtifact(it.url, Hash.NONE))
+                val purlType = urlToPackageType(it.url).toString()
 
                 // TODO: FossID doesn't return the line numbers of the match, only the character range. One must use
                 //       another call "getMatchedLine" to retrieve the matched line numbers. Unfortunately, this is a