net.ext_grid.controllable is nan for "1-MV-semiurb--0-sw"

[KS-HTK]

for the simbench network "1-MV-semiurb--0-sw" the ext_grid.controllable column contains invalid value.

This was found due to #1101 in pandapower

Example:

from simbench import get_simbench_net

net = get_simbench_net("1-MV-semiurb--0-sw")
print(net.ext_grid.controllable)

returns NaN, but the controllable column should only ever contain True/False (bool).