Add additional check for ZipAlign task

In case when in `build.gradle` we have something like this:
```
signingConfigs {
        release {
           // configuration for sign release build like (keystore, alias, passwords etc.)
           // which permitted only for few publishers and added to .gitgnore (due to security reasons)
        }
        debug {
            // Use same debug certificate within team
            storeFile rootProject.file("debug.keystore")
        }
    }
```
team members without release signing configs will receive error:
```
Caused by: java.lang.NullPointerException: Cannot get property 'outputFile' on null object
	at de.triplet.gradle.play.PlayPublisherPlugin$_apply_closure2.doCall(PlayPublisherPlugin.groovy:83)
```
because `zipAlignEnabled == True` (by default for release sign config), but `outputData.zipAlign == null` because the task can be null if the variant is a library or if the APK cannot be signed.