Android: Don't set the signingConfig if keystore property isn't set

If the property isn't set, we don't initialize the release config, so we shouldn't use it. This fixes building issues for me.

Source/Android/app/build.gradle

@@ -48,7 +48,9 @@ android {
     buildTypes {
         // Signed by release key, allowing for upload to Play Store.
         release {
-            signingConfig signingConfigs.release
+            if (project.hasProperty('keystore')) {
+                signingConfig signingConfigs.release
+            }
 
             minifyEnabled true
             shrinkResources true