[PackageModel] SupportedFeatures: Handle absence of optional features section

`JSON.get` would throw if the key couldn't be found, let's handle
that gracefully for an optional "optional features" section.

(cherry picked from commit 4c964b1)

Author: xedin

Sources/PackageModel/Toolchain+SupportedFeatures.swift

@@ -102,7 +102,9 @@ extension Toolchain {
 
             let features: JSON = try parsedSupportedFeatures.get("features")
 
-            let optional: [SwiftCompilerFeature] = try (features.get("optional") as [JSON]?)?.map { (json: JSON) in
+            let optionalFeatures = (try? features.getArray("optional")) ?? []
+
+            let optional: [SwiftCompilerFeature] = try optionalFeatures.map { json in
                 let name: String = try json.get("name")
                 let categories: [String]? = try json.getArrayIfAvailable("categories")
                 let migratable: Bool? = json.get("migratable")
@@ -114,7 +116,7 @@ extension Toolchain {
                     categories: categories ?? [name],
                     flagName: flagName
                 )
-            } ?? []
+            }
 
             let upcoming: [SwiftCompilerFeature] = try features.getArray("upcoming").map {
                 let name: String = try $0.get("name")