May 2021 Update (1.31.0)

• GitHub workflow updates
• Updates to PHPDocs for v1 and beta models
• Return null when a property of a model is null as opposed to initialising a model with null or initialising a built-in PHP type e.g. DateTime
• Updating code indentation in generated models
• Fix missing \Beta\Microsoft\Graph\Model\Single and \Microsoft\Graph\Model\Single models by casting OData Single type to float in generated models