In maxGraph0.8., When this.connectionHandler.errorequals empty string，this.connectionHandler.error && this.connectionHandler.error.length === 0equals false

But in the old version, the value of this expression（this.error != null && this.error.length == 0） was true

This is the result of the expression running on the console，the result is true

I made a fix in my project to keep the original functionality. If Return values for validateConnection is an empty string, I will use an empty array instead.

this.graph.getPlugin('ConnectionHandler').validateConnection = function(source, target) {
let error = null;
if (!this.isValidTarget(target)) {
error = '';
}
error = this.graph.getEdgeValidationError(null, source, target);
return error === '' ? [] : error;
}