Don't allow a null dictionary to be stored.

src/Features/Core/Portable/FindUsages/DefinitionItem.cs

@@ -86,7 +86,7 @@ internal abstract partial class DefinitionItem
             NameDisplayParts = nameDisplayParts.IsDefaultOrEmpty ? displayParts : nameDisplayParts;
             OriginationParts = originationParts.NullToEmpty();
             SourceSpans = sourceSpans.NullToEmpty();
-            Properties = properties;
+            Properties = properties ?? ImmutableDictionary<string, string>.Empty;
             DisplayIfNoReferences = displayIfNoReferences;
         }