Provided the class attribute is present as you say you can use the attribute selector like this:
jsFiddle
asd
a[class=""] {
color: red;
}
If you want this to work when there is no class attribute present on the element you can use :not([class]).
jsFiddle
asd
a:not([class]) {
color: red;
}
These can then be combined together to handle both cases.
jsFiddle
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asd
a[class=""],
a:not([class]) {
color: red;
}