Don't coerce nil for collection attributes when required is false

Without this fix, collection attributes can never be set to nil when
coercion is enabled (because the coercer coerces nil to an empty array).

lib/virtus/attribute/collection.rb

@@ -71,7 +71,7 @@ def self.merge_options!(type, options)
 
       # @api public
       def coerce(value)
-        coerced = super
+        coerced = value.nil? && !required? ? nil : super
 
         return coerced unless coerced.respond_to?(:each_with_object)
 

spec/unit/virtus/attribute/collection/coerce_spec.rb

@@ -61,14 +61,27 @@
 
     let(:object) {
       described_class.build(
-        Array[member_primitive], coercer: coercer, member_type: member_type
+        Array[member_primitive], coercer: coercer, member_type: member_type, required: required
       )
     }
 
-    it 'returns nil' do
-      mock(coercer).call(input) { input }
+    context 'when required' do
+      let(:required) { true }
+      let(:output)   { Array(input) }
 
-      expect(subject).to be(input)
+      it 'returns an empty array' do
+        mock(coercer).call(input) { output }
+
+        expect(subject).to eq(output)
+      end
+    end
+
+    context 'when not required' do
+      let(:required) { false }
+
+      it 'returns nil' do
+        expect(subject).to be(input)
+      end
     end
   end
 end