complex_plot: fix plot of undefined values

Undefined values would get some random color.
Rreplace all undefined values by white.

In numpy 1.24 this avoids a runtime warning (cast of nan to uint8).

Author: tornaria

src/sage/plot/complex_plot.pyx

@@ -461,6 +461,8 @@ def complex_to_rgb(z_values, contoured=False, tiled=False,
             rgb[i, j, 2] = b
 
     sig_off()
+    nan_indices = np.isnan(rgb).any(-1)     # Mask for undefined points
+    rgb[nan_indices] = 1                    # Make nan_indices white
     return rgb