refactor: make implementation param/return non optional

Author: jcesarmobile

assets/plugin-template/android/src/main/kotlin/__JAVA_PATH__.kt.mustache

@@ -4,8 +4,8 @@ import android.util.Log
 
 class {{ CLASS }} {
 
-    fun echo(value: String?): String? {
-        Log.i("Echo", value ?: "null")
+    fun echo(value: String): String {
+        Log.i("Echo", value)
 
         return value
     }

assets/plugin-template/android/src/main/kotlin/__JAVA_PATH__Plugin.kt.mustache

@@ -13,7 +13,7 @@ class {{ CLASS }}Plugin : Plugin() {
 
     @PluginMethod
     fun echo(call: PluginCall) {
-        val value = call.getString("value")
+        val value = call.getString("value") ?: ""
 
         val ret = JSObject().apply {
             put("value", implementation.echo(value))