Fix bug on GetDataLimits when plot is empty

[EFeru]

When plot is empty meaning no plottable in GetPlottables() then the returned limits have the initial values:

ScottPlot/src/ScottPlot/Plot/Plot.Axis.cs

Lines 355 to 358 in a214b94

	double xMin = double.PositiveInfinity;
	double xMax = double.NegativeInfinity;
	double yMin = double.PositiveInfinity;
	double yMax = double.NegativeInfinity;

Now, if the user calls:

AxisLimits limits = formsPlot1.Plot.GetDataLimits();
formsPlot1.Plot.SetOuterViewLimits(limits.XMin, limits.XMax, limits.YMin, limits.YMax);

if is not possible to interract with the plot anymore (zoom, pan, etc.) beacause the limits set are flipped. This PR fixes the returned limits to the correct ones.

[swharden]

Good catch @EFeru!

Re-stating the problem to remind my future self, the GetDataLimits() method works by initially setting limits to extreme inverted values with the expectation that data points from plottables will pull them into an expected range

ScottPlot/src/ScottPlot/Plot/Plot.Axis.cs

Lines 353 to 358 in a214b94

	public AxisLimits GetDataLimits(int xAxisIndex = 0, int yAxisIndex = 0)
	{
	double xMin = double.PositiveInfinity;
	double xMax = double.NegativeInfinity;
	double yMin = double.PositiveInfinity;
	double yMax = double.NegativeInfinity;

...but if there are no plottables with data, the returned limits are wacky and inverted.

This PR detects if the initial values were never changed (as they would be expected to by data) and sets them to the opposite value