fix PETScKrylovSolver.__call__() for no initial guess
fix PETScKrylovSolver.__call__() for no initial guess

sfepy/sfepy,rc/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy,vlukes/sfepy,lokik/sfepy,lokik/sfepy,lokik/sfepy,sfepy/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,lokik/sfepy,rc/sfepy