Prevent a NullPointerException

The regression-corpus contains some documents which trigger this,
but succeed with the added null-check

poi-scratchpad/src/main/java/org/apache/poi/hwmf/draw/HwmfGraphics.java

@@ -629,7 +629,7 @@ public class HwmfGraphics implements HwmfCharsetAware {
         }
 
         FontCharset fc = font.getCharset();
-        if (fc == FontCharset.DEFAULT) {
+        if (fc == FontCharset.DEFAULT || fc == null) {
             return charsetProvider.get();
         }