toasty/fits_tiler.py: check if hips_{pixel_cut,data_range} are present

In one test image I have, hipsgen produces a "BAD PIXEL CUT" warning and
doesn't emit this information. (I think it's because the file has an
integer data type.) Avoid the crash if this is the case.

toasty/fits_tiler.py

@@ -388,11 +388,19 @@ class FitsTiler(object):
         self.builder.imgset.base_degrees_per_tile = float(
             hips_properties["hips_initial_fov"]
         )
-        pixel_cut = hips_properties["hips_pixel_cut"].split(" ")
-        self.builder.imgset.pixel_cut_low = float(pixel_cut[0])
-        self.builder.imgset.pixel_cut_high = float(pixel_cut[1])
-        data_range = hips_properties["hips_data_range"].split(" ")
-        self.builder.imgset.data_min = float(data_range[0])
-        self.builder.imgset.data_max = float(data_range[1])
+
+        pixel_cut = hips_properties.get("hips_pixel_cut")
+
+        if pixel_cut is not None:
+            pixel_cut = pixel_cut.split(" ")
+            self.builder.imgset.pixel_cut_low = float(pixel_cut[0])
+            self.builder.imgset.pixel_cut_high = float(pixel_cut[1])
+
+        data_range = hips_properties.get("hips_data_range")
+
+        if data_range is not None:
+            data_range = data_range.split(" ")
+            self.builder.imgset.data_min = float(data_range[0])
+            self.builder.imgset.data_max = float(data_range[1])
 
         self.builder.imgset.url = "Norder{0}/Dir{1}/Npix{2}"