Fix bug that stops Figure.coast from plotting with only dcw (#910)
Fix bug that stops Figure.coast from plotting with only dcw (#910)

GMT allows coast to plot with the `-E` argument and no other colors set,
but PyGMT would raise an error if `dcw` was the only argument.
This is a fix for that.
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python