How to interpret op amp minimum supply voltage if the datasheet field is blank?

The datasheet seems to have lost some coherence in the translation from NS to TI. If you look at an original datasheet, it's clear that the recommended voltage is 15V for the LM356N and the maximum is +/-18V.

It's not clear what the recommended minimum is, however it's specified at +/-15V. Curves of typical performance go down to +/-5V so probably you're okay down to 10V (obviously with reduced output swing and input range).

As mentioned, it looks like the data sheet lacks a bit.

If nothing else, look at the output voltage swing. It's listed as \$\pm\$10V for a 2k\$\Omega\$ load and \$\pm\$15V supply. That means that the output is dropping 5V in each direction -- so if you give it a 2k\$\Omega\$ load and a \$\pm\$5V supply it'll have barely any output swing at all.