target_dependency: Fix crash if target_proxy is nil.

If `target_proxy` is `nil`, calling `to_tree_hash` on it will raise an
exception: undefined method `to_tree_hash' for nil:NilClass
(NoMethodError).

lib/xcodeproj/project/object/target_dependency.rb

@@ -80,7 +80,7 @@ def to_tree_hash
           hash = {}
           hash['displayName'] = display_name
           hash['isa'] = isa
-          hash['targetProxy'] = target_proxy.to_tree_hash
+          hash['targetProxy'] = target_proxy.to_tree_hash if target_proxy
           hash
         end
 

spec/project/object/target_dependency_spec.rb

@@ -66,6 +66,17 @@ module ProjectSpecs
       }
     end
 
+    it 'tree hash for target dependency without target proxy' do
+      target = @project.new_target(:static, 'Pods', :ios)
+      @target_dependency.target = target
+      target.dependencies << @target_dependency
+
+      @target_dependency.to_tree_hash.should == {
+        'displayName' => 'Pods',
+        'isa' => 'PBXTargetDependency',
+      }
+    end
+
     #----------------------------------------#
 
     describe '#display_name' do