Ensure nodata detection is more robust in raster builders

[martinfleis]

As pointed out in #740, the only case when our raster contiguity builders capture nodata information is when that is encoded in da.attrs["nodatavals"]. I am not sure when that happens but I've seen more DataArrays where missing is encoded differently. Usually, da.rio.nodata can pick that but on float array, you can also have np.nan etc. We shall revise this logic and make it a bit more robust so a user does not have to manually add nodatavals to attrs before building the graph.