Only add FMLCorePluginContainsFMLMod to MANIFEST if core mod is actually used

build.gradle

@@ -359,7 +359,7 @@ processResources
 
 def getManifestAttributes() {
     def manifestAttributes = [:]
-    if(containsMixinsAndOrCoreModOnly.toBoolean() == false) {
+    if(containsMixinsAndOrCoreModOnly.toBoolean() == false && (usesMixins.toBoolean() || coreModClass) {
         manifestAttributes += ["FMLCorePluginContainsFMLMod": true]
     }