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

[Pokechu22]

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.

dolphin/Source/Android/app/build.gradle

Lines 36 to 45 in 7d2d5d9

	signingConfigs {
	release {
	if (project.hasProperty('keystore')) {
	storeFile file(project.property('keystore'))
	storePassword project.property('storepass')
	keyAlias project.property('keyalias')
	keyPassword project.property('keypass')
	}
	}
	}

Before, I was getting this message when running gradlew assemble:

> Task :app:packageRelease FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:packageRelease'.
> A failure occurred while executing com.android.build.gradle.tasks.PackageAndroidArtifact$IncrementalSplitterRunnable
   > SigningConfig "release" is missing required property "storeFile".

I haven't tested this change in a setup which has an actual signing config (and don't really want to set one up myself).