Added handler for properties with null value when inserting (#89)

Fixes #88

* Added handler for properties with null value when inserting

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added test for checking ignored attach with invalid encoding

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Removed test

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added test for unknown properties with null value

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Optimize imports

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Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>