Could minLength=1 be used for all string attributes in the OpenAPI specs, except for the cases where it clearly makes sense to differentiate between "not provided" and "empty".

The string types (such as Max35Text, Max70Text etc.) in the OpenAPI specs differ from the ISO 20022 standard types of the same names, where min length is prescribed to be 1.
The VOP API WB agreed on the proposition as it is ISO standard. 

The decision has been taken to include this point in a future version of the published YAML file.