[Rd] image() method for Matrix fails on empty matrices (?)

[Ben Bolker]

 Reasonably easy to avoid, but maybe an edge case that should be
handled?  Haven't looked yet to see how easy it would be to fix ... Am I
missing something?

> library(Matrix)
> m <- Matrix(0,nrow=3,ncol=3)
> m
3 x 3 sparse Matrix of class "dsCMatrix"

[1,] . . .
[2,] . . .
[3,] . . .
> image(m)
Error in seq.default(zrng[1], zrng[2], length.out = cuts + 2) :
  'from' must be a finite number
In addition: Warning messages:
1: In min(xx, na.rm = TRUE) :
  no non-missing arguments to min; returning Inf
2: In min(x) : no non-missing arguments to min; returning Inf
3: In max(x) : no non-missing arguments to max; returning -Inf
4: In min(x) : no non-missing arguments to min; returning Inf
5: In max(x) : no non-missing arguments to max; returning -Inf

## now modify matrix
> m[3,3] <- 1
> image(m) ## works fine
## reset:
> m[3,3] <- 0
> image(m) ## fails again