  Self-consistent proton-neutron quasiparticle random phase approximation based
on the spherical nonlinear point-coupling relativistic Hartree-Bogoliubov
theory is established and used to investigate the $\beta^+$/EC-decay half-lives
of neutron-deficient Ar, Ca, Ti, Fe, Ni, Zn, Cd, and Sn isotopes. The isoscalar
proton-neutron pairing is found to play an important role in reducing the decay
half-lives, which is consistent with the same mechanism in the $\beta$ decays
of neutron-rich nuclei. The experimental $\beta^+$/EC-decay half-lives can be
well reproduced by a universal isoscalar proton-neutron pairing strength.
