Support fully-zero dash arrays.

src/_mplcairo.cpp

@@ -626,7 +626,14 @@ void GraphicsContextRenderer::set_dashes(
     for (auto i = 0; i < n; ++i) {
       buf[i] = points_to_pixels(dashes_raw[i]);
     }
-    cairo_set_dash(cr_, buf.get(), n, points_to_pixels(*dash_offset));
+    if (std::all_of(buf.get(), buf.get() + n, std::logical_not{})) {
+      // Treat fully zero dash arrays as solid stroke.  This is consistent with
+      // the SVG spec (PDF and PS specs explicitly reject such arrays), and
+      // also generated by Matplotlib for zero-width lines due to dash scaling.
+      cairo_set_dash(cr_, nullptr, 0, 0);
+    } else {
+      cairo_set_dash(cr_, buf.get(), n, points_to_pixels(*dash_offset));
+    }
   } else {
     cairo_set_dash(cr_, nullptr, 0, 0);
   }