Sometimes it's not desired to have a graphical signature, e.g. when there is not free space in the page to contain the rectangle for the graphical signature, even small. Indeed [I think] in PDF "official" standard the graphical signature is NOT a must. One solution is to double click on a any point inside page, but maybe it's not intuitive for general users. Another better option would be to include an option (in Preferences > Configure Okular) with a check to enable or disable.