Prefer non-optional UTF8 String <-> Data conversion.

Hopefully addresses the swift-lint errors.

Fix via realm/SwiftLint#5263

components/nimbus/ios/Nimbus/Dictionary+.swift

@@ -7,10 +7,7 @@ import Foundation
 extension Dictionary where Key == String, Value == Any {
     func stringify() throws -> String {
         let data = try JSONSerialization.data(withJSONObject: self)
-        guard let s = String(data: data, encoding: .utf8) else {
-            throw NimbusError.JsonError(message: "Unable to encode")
-        }
-        return s
+        return String(decoding: data, as: UTF8.self)
     }
 
     static func parse(jsonString string: String) throws -> [String: Any] {

components/nimbus/ios/Nimbus/Nimbus.swift

@@ -412,7 +412,7 @@ extension Nimbus: NimbusMessagingProtocol {
         encoder.keyEncodingStrategy = .convertToSnakeCase
 
         let data = try encoder.encode(additionalContext)
-        let string = String(data: data, encoding: .utf8)!
+        let string = String(decoding: data, as: UTF8.self)
         return try createMessageHelper(string: string)
     }