Avoid NPE with @GeneratedInjector with Provider(null)

[matanlurey]

From an internal customer:

SEVERE]: Unhandled exception in the AngularDart compiler!
 
Please report a bug: go/angular-dart-bug
Unsupported operation: Null
package:source_gen/src/constants/reader.dart 133:33                                                                                           _NullConstant.objectValue
package:angular_compiler/src/analyzer/di/providers.dart 52:28                                                    ProviderReader._parseProvider

... we should be careful and check (and error "nicely" for null in this case).