Fixed bug in VectorFieldPlot magnitude normalization
Fixed bug in VectorFieldPlot magnitude normalization

Division by zero error was raised when VectorField had zero magnitude,
normalization now skipped.

basnijholt/holoviews,mjabri/holoviews,ioam/holoviews,mjabri/holoviews,mjabri/holoviews,vascotenner/holoviews,ioam/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews,basnijholt/holoviews,vascotenner/holoviews