Do not serialize empty collections (#30)

This is a temporary workaround before aaubry/YamlDotNet#608 gets merged.

This makes the YAML not contain bunch of rows like this:
```
variables: []
```

src/Sharpliner/GraphTraversal/SharplinerGraphTraversalStrategy.cs

@@ -0,0 +1,49 @@
+using YamlDotNet.Serialization;
+using System.Collections.Generic;
+using System.Collections;
+using YamlDotNet.Serialization.ObjectGraphTraversalStrategies;
+
+namespace Sharpliner.GraphTraversal
+{
+    /// <summary>
+    /// This is a workaround that skips empty collections and doesn't serialize them at all.
+    /// The proper way to do this is in a PR here: https://github.com/aaubry/YamlDotNet/pull/608
+    /// Before that is fixed, this is how we're going to do things (╯°□°）╯︵ ┻━┻
+    /// </summary>
+    public class SharplinerGraphTraversalStrategy : FullObjectGraphTraversalStrategy
+    {
+        private readonly ITypeInspector _typeDescriptor;
+
+        public SharplinerGraphTraversalStrategy(ITypeInspector typeDescriptor, ITypeResolver typeResolver, int maxRecursion, INamingConvention namingConvention)
+            : base(typeDescriptor, typeResolver, maxRecursion, namingConvention)
+        {
+            _typeDescriptor = typeDescriptor;
+        }
+
+        protected override void TraverseProperties<TContext>(IObjectDescriptor value, IObjectGraphVisitor<TContext> visitor, TContext context, Stack<ObjectPathSegment> path)
+        {
+            visitor.VisitMappingStart(value, typeof(string), typeof(object), context);
+
+            var source = value.NonNullValue();
+            foreach (var propertyDescriptor in _typeDescriptor.GetProperties(value.Type, source))
+            {
+                var propertyValue = propertyDescriptor.Read(source);
+
+                if (visitor.EnterMapping(propertyDescriptor, propertyValue, context))
+                {
+                    // This is the actual change
+                    if (propertyValue.Value is IEnumerable collection && !collection.GetEnumerator().MoveNext())
+                    {
+                        // Skip empty arrays/collections
+                        continue;
+                    }
+
+                    Traverse(propertyDescriptor.Name, new ObjectDescriptor(propertyDescriptor.Name, typeof(string), typeof(string)), visitor, context, path);
+                    Traverse(propertyDescriptor.Name, propertyValue, visitor, context, path);
+                }
+            }
+
+            visitor.VisitMappingEnd(value, context);
+        }
+    }
+}

src/Sharpliner/SharplinerSerializer.cs

@@ -1,5 +1,6 @@
 using YamlDotNet.Serialization.NamingConventions;
 using YamlDotNet.Serialization;
+using Sharpliner.GraphTraversal;
 
 namespace Sharpliner.Definition
 {
@@ -13,6 +14,7 @@ private static ISerializer InitializeSerializer()
         {
             var serializerBuilder = new SerializerBuilder()
                 .WithNamingConvention(CamelCaseNamingConvention.Instance)
+                .WithObjectGraphTraversalStrategyFactory((typeInspector, typeResolver, typeConverters, maximumRecursion) => new SharplinerGraphTraversalStrategy(typeInspector, typeResolver, maximumRecursion, CamelCaseNamingConvention.Instance))
                 .ConfigureDefaultValuesHandling(DefaultValuesHandling.OmitDefaults);
 
             return serializerBuilder.Build();

tests/Sharpliner.Tests/AzureDevOps/PipelineSerializationTests.cs

@@ -65,7 +65,6 @@ public void Serialize_Pipeline_Test()
     - name: AzureSubscription
       value: Prod
     - group: azure-prod
-stages: []
 ");
         }