fix pbxProject.addBuildPhase

[Athorcis]

addBuildPhase method fail when pbx file contains file references without path, it happen with cordova-ios 7. The file reference without path is [Object: null prototype] { isa: 'PBXFileReference', lastKnownFileType: 'folder.assetcatalog', name: 'Assets.xcassets', sourceTree: '"<group>"' }