Fix NoMethodError when diffing manifest for new apps with features (#4941)

The remove_unspecified_app_features method was called without checking
if existing_value was present, causing a NoMethodError when applying
a manifest with features for a new (non-existing) app:

  undefined method `slice!' for nil:NilClass
  .../space_diff_manifest.rb:220:in `remove_unspecified_app_features'

Author: philippthun

app/actions/space_diff_manifest.rb

@@ -51,7 +51,7 @@ def generate_diff(app_manifests, space)
 
             remove_default_missing_fields(existing_value, key, value) if nested_attribute_exists
 
-            remove_unspecified_app_features(existing_value, key, value)
+            remove_unspecified_app_features(existing_value, key, value) if existing_value.present?
 
             # To preserve backwards compability, we've decided to skip diffs that satisfy this conditon
             next if !nested_attribute_exists && %w[disk_quota disk-quota memory].include?(key)

spec/unit/actions/space_diff_manifest_spec.rb

@@ -251,15 +251,19 @@ module VCAP::CloudController
               'name' => 'new-app'
             },
             {
-              'name' => 'newer-app'
+              'name' => 'newer-app',
+              'features' => {
+                'ssh' => true
+              }
             }
           ]
         end
 
         it 'returns the correct diff' do
           expect(subject).to eq([
             { 'op' => 'add', 'path' => '/applications/0/name', 'value' => 'new-app' },
-            { 'op' => 'add', 'path' => '/applications/1/name', 'value' => 'newer-app' }
+            { 'op' => 'add', 'path' => '/applications/1/name', 'value' => 'newer-app' },
+            { 'op' => 'add', 'path' => '/applications/1/features', 'value' => { 'ssh' => true } }
           ])
         end
       end