Only use signing configuration if env variables are defined

uhabits-android/build.gradle

@@ -16,6 +16,7 @@ android {
     compileSdkVersion COMPILE_SDK_VERSION as Integer
     buildToolsVersion BUILD_TOOLS_VERSION
 
+    if(project.hasProperty("LOOP_STORE_FILE")) {
         signingConfigs {
             release {
                 storeFile file(LOOP_STORE_FILE)
@@ -24,6 +25,8 @@ android {
                 keyPassword LOOP_KEY_PASSWORD
             }
         }
+        buildTypes.release.signingConfig signingConfigs.release
+    }
 
     playAccountConfigs {
         defaultAccountConfig {
@@ -53,7 +56,6 @@ android {
         release {
             minifyEnabled true
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
-            signingConfig signingConfigs.release
         }
 
         debug {