null string for compiling

/* making .display-block */
.make-classes () {
    @item : ~"";
    .make-classes( @item );
}

/* making .xs-display-block */
.make-classes ( @class ) when ( iskeyword( @class ) ) {
    @item : ~"@{class}-";
    .make-classes( @item );
}

/* inner function */
.make-classes ( @class ) when ( isstring( @class ) ) {
    .@{class}display-block { display: block; }
}

.make-classes ();

.make-classes( xs );

If I want to taking .display-block into some class as :

.shi {
    .display-block;
}

it will be tell me

NameError: .display-block is undefined on line 21, column 5:

but if I using .xs-display-block

.shi {
    .xs-display-block;
}

It's works.

so... Is that the null string are not supporting when compiling?

Start by compiling the provided Less reproduction and compare the generated .display-block and .xs-display-block mixins. Investigate how the compiler handles the empty string argument versus the keyword argument; done means the reported behavior is explained and the reproduction has a verified expected result.