[NTG-context] 'empty string' test in MP

Hi all, I pass a string to a MP graphic using \setMPtext{LABEL}{TEXT}. The string can also be empty, and then no action (for the text) is taken. How can I test in MP, if the given \MPstring{LABEL} is empty? I have tried everything I can imagine in the last one hour, but no success. Not my day... This is the old, not working version if not(\MPstring{LABEL}="") : % is always true. Peter