Ensure we deal correctly with linear problems when N.init ~= [].

Previously, we would obtain weird results for example
```
dom = [0,1];
N = chebop(@(x,u) 0.02.*diff(u,2)+diff(u)+u, dom);
rhs = 0;
N.bc = @(x,u) [u(0); u(1)-1];
x = chebfun(@(x) x, dom);
N.init =  sin(10*x);
options = cheboppref();
options.discretization = 'collocation';
u = solvebvp(N, rhs, options)
plot(u, 'LineWidth', 2)
```