how does Breeze saves empty strings

I am retrieving an entity from the database with some proprieties.

One of the proprieties is not allowed to be not null string in the database. Even though is not null it can be an empty string "".

In the EF mappings the propriety is validated like:

this.Property(t => t.ColumnName)
            .IsRequired()
            .HasMaxLength(50);

The problem is that when I am trying to save the changes with Breeze it returns an error for the elements that have the propriety ColumnName equal to an empty string.(saying "ColumnName is required").

Is this accepted behaviour? Shouldn't the error be thrown only if the the ColumnName would be null or undefined?

The error Breezejs throws:

valError: Error
    entityErrors: Array[5]
        0: Object
            entity: Object
                ColumnName: function dependentObservable() {
                    __ko_proto__: function (evaluatorFunctionOrOptions, evaluatorFunctionTarget, options) {
                    _latestValue: ""
                    _subscriptions: Object
                    ...
                ...
                entityAspect: ctor
                __proto__: Object
            errorMessage: "'ColumnName' is required"
            errorName: "required"
            isServerError: false
            propertyName: "ColumnName"
            __proto__: Object

This has nothing to do with how Breeze saves data.

What you are encountering is Breeze's validation logic. Breeze defaults to treating empty strings as nulls for the purposes of validating "required" fields. You can change this by replacing Breeze's required validator to simply treat required as meaning "not null".

Validator.required = function (context) {
    var valFn = function (v, ctx) {
        return v != null;
    }
    return new Validator("required", valFn, context);
};
// register the new validator so that metadata can find it. 
Validator.registerFactory(Validator.required, "required");