Default PropertyDefinition Type field to UNDEFINED

[jwillp]

Because of this line, we get an exception thrown, when someone does not specify a type on a property:

ObjectGenerator/src/Domain/Definition/Schema/PropertiesDefinitionSchema.php

Line 21 in b6442fd

->scalarNode(SchemaKey::TYPE)->defaultValue(null)->end()

Since it defaults to null, if someone forgets to put a type in their definition,
instead of being handled later as mixed (as it should be) it throws an exception at :

ObjectGenerator/src/Domain/Definition/PropertyDefinition.php

Line 191 in b6442fd

$propertyDefinition->setType(VariableType::fromString($data[SchemaKey::TYPE]));

I suspect, the fix would be to change the defaultValue null to VariableType::UNDEFINED.