Fix builds for Swift 5.6 and below

Sources/Where.swift

@@ -197,7 +197,8 @@ public struct Where<O: DynamicObject>: WhereClauseType, FetchClause, QueryClause
             )
         }
         switch value {
-            
+
+#if swift(>=5.7)
         case let optionalValue as any FieldOptionalType:
             switch optionalValue.cs_wrappedValue {
                 
@@ -209,6 +210,12 @@ public struct Where<O: DynamicObject>: WhereClauseType, FetchClause, QueryClause
                 self.init(valuePredicate)
             }
 
+#else
+        case nil:
+            self.init(nilPredicate)
+
+#endif
+
         case is NSNull:
             self.init(nilPredicate)