fix deprecation warnings beign repeated if the mod can't be identified

release-notes.md

@@ -4,6 +4,8 @@
 See [log](https://github.com/CLxS/SMAPI/compare/1.1...master).
 
 This is a patch release that addresses compatibility with older mods.
+
+* Fixed deprecation warnings being repeated if the mod can't be identified.
 ## 1.1
 See [log](https://github.com/CLxS/SMAPI/compare/1.0...1.1).
 

src/StardewModdingAPI/Framework/DeprecationManager.cs

@@ -49,7 +49,7 @@ namespace StardewModdingAPI.Framework
         public void Warn(string source, string nounPhrase, string version, DeprecationLevel severity)
         {
             // ignore if already warned
-            if (source != null && !this.MarkWarned(source, nounPhrase, version))
+            if (!this.MarkWarned(source ?? "<unknown>", nounPhrase, version))
                 return;
 
             // build message