Color polygons with NA as white with black borders.

nagpd.py

fig, ax = plt.subplots(figsize=(10, 8), tight_layout=True)
ax.set_title("SAM claims by county", fontsize=10)
dd.plot(
    ax=ax, column="n", edgecolor="black", linewidth=.50, 
    cmap="rainbow", alpha=1, legend=True,
    legend_kwds={"label": "number of claims", "orientation": "vertical", "shrink": .35},
    missing_kwds={"facecolor": "#FFFFFF", "edgecolor": "#000000", "linewidth": .25}
    )
ax.axis("off")
plt.show()