Fix pen_width bug in cairo_draw

When user passes pen_width value only for either vertex or edge, an
error is thrown since inside min_lw, lw may be None. This bug fixes
that by assigning value lw=0.1 whenever it is 0.1

src/graph_tool/draw/cairo_draw.py

@@ -1339,6 +1339,8 @@ def fit_to_view_ink(g, pos, output_size, vprops, eprops, adjust_aspect=False,
             x[x < 0.05] = 0.1
             lw.fa = x
         else:
+            if lw == None:
+                lw = 0.1
             lw = max(lw, 0.1)
         return lw
     vprops = dict(vprops, pen_width=min_lw(vprops.get("pen_width")))