Hello there,
if you use File Reader node might be it will work out of the box and if not there is Missing Value Pattern under Advanced....
Although think this should be addressed in reader nodes one can use regexReplace() function with same outcome in such/similar cases:
toNull( regexReplace( $$CURRENTCOLUMN$$, "m" , "") )
Welcome to KNIME Community @arne_g99!
Br,
Ivan