Improve zap gen template for Matter access privilege definitions

[mlepage-google]

PR #16327 added zap gen with src/app/zap-templates/templates/app/access.zapt performing a series of nested checks:

           {{#if hasOperation}}
                {{#if (isStrEqual operation "read")}}
                    {{#if hasRole}}
                        {{#if (isStrEqual role "view")}}

The hasOperation check is necessary because the operation property is null if it's not present. Likewise for the role property.

These checks would be simpler if the properties were simply an empty string, instead of null.

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