GeneratedTypeBuilderImpl allows add property with null name. The Preconditions.checkArgument(name != null,"...") test is missing in the method addProperty(String name) in the class AbstractGeneratedTypeBuilder.

Example:

GeneratedTypeBuilderImpl generatedTypeBuilder = new GeneratedTypeBuilderImpl(
"my.package", "MyName");

GeneratedPropertyBuilder propertyBuilderNull = generatedTypeBuilder.addProperty(null);

propertyBuilderNull != null //true

GeneratedType instance = generatedTypeBuilder.toInstance();
List<GeneratedProperty> properties = instance.getProperties();

properties.size() //1
properties.contains(propertyBuilderNull.toInstance(instance)) //true

Notice: The containsProperty method have this check:
public boolean containsProperty(final String name) {
Preconditions.checkArgument(name != null, "Parameter name can't be null");