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Handle nullable attributes in TLV-to-attr-store interactions and Acce…
…ssors (project-chip#11665) * Add a way to ask EmberAfAttributeMetadata whether the attribute is nullable. * Convert ZCL null values to/from TLV null when attributes are read/written. This implements the following semantics, for attributes we store in the attribute store (integers, booleans, strings, octet strings): Writes: * An attempt to write TLV null for non-nullable attributes leads to an error. * Writing TLV null to a nullable string or octet string sets its length to 0xFFFF or 0xFF depending on whether it's a long string or not. * Writing TLV null to a nullable integer or boolean uses the "NaU" or "NaS" ZCL representation to represent it. * Writing a TLV integer to an integer attribute makes a CanRepresentValue check and returns an error if it returns false. For now CanRepresentValue only disallows the NaU/NaS value for nullable attributes. Reads: * When reading a nullable integer or boolean, NaU/NaS is converted to TLV null. * When reading an integer or boolean all other cases go through a CanRepresentValue check (in case the value in the attr store is not actually valid) and return error if it returns false. * When reading a nullable string or octet string, length 0xFFFF/0xFF is converted to TLV null. * When reading a non-nullable string or octet string, length 0xFFFF/0xFF leads to an error. * Fix Accessors to handle nullable attributes. For setters, the new behavior is: * For non-string attributes when setting an integer or boolean value, do a CanRepresentValue check and fail if that fails. For string attributes, ensure that strings with 0xFF or 0xFFFF as length cannot be passed in in practice. * For nullable attributes add a SetNull method. * For nullable attributes add a Set() method taking a Nullable<T> and calling either SetNull or the setter that expects a non-null value. For getters, the new behavior is: * When reading a nullable integer or boolean, NaU/NaS is converted to a null value stored in the Nullable. * When reading a non-nullable integer or boolean, return error if CanRepresentValue returns false on the value from the attr store. * When reading a nullable string or octet string, length 0xFFFF/0xFF is converted to a null value stored in the Nullable. * When reading a non-nullable string or octet string, length 0xFFFF/0xFF leads to an error. The XML changes are for two problems the new asserts caught: we had strings that were short-typed, but had sizes that would not fit in a single byte length. * Add tests for nullable attributes. * Fix the Darwin tests by skipping the nullable read for now * Address review comments
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