[All][Consistency] Shared model should not throw exceptions for invalid adaptive cards

[jwoo-msft]

Update parsing behaviors such that exception is not thrown when empty or null value is given for a property even when the property "required". For example, If value for url of Image is missing or empty, parsing the string for url should not yield an exception by the parsers. Elements should be dropped by the parser in this case.

For more detailed procedure of handling improper values. please see here

See also: #4565

[ghost]

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