Make keystore properties cast nullable

In groovy there is no need to cast to string but in kotlin you cast keystoreProperties["XXXX"] to String which fail on null, there is two solution force every user who want to contribute to have a keystore or allow nullable keystoreProperties.
I went with the latter to limit contributor friction.

Author: Renarde-dev

android/app/build.gradle.kts

@@ -45,10 +45,10 @@ android {
 
     signingConfigs {
         create("release") {
-            keyAlias = keystoreProperties["keyAlias"] as String
-            keyPassword = keystoreProperties["keyPassword"] as String
+            keyAlias = keystoreProperties["keyAlias"] as? String
+            keyPassword = keystoreProperties["keyPassword"] as? String
             storeFile = keystoreProperties["storeFile"]?.let { file(it) }
-            storePassword = keystoreProperties["storePassword"] as String
+            storePassword = keystoreProperties["storePassword"] as? String
         }
     }