Prevent crash

app/src/main/java/eu/faircode/email/HtmlHelper.java

@@ -1934,6 +1934,7 @@ public class HtmlHelper {
                             String value = param.substring(semi + 1);
                             switch (key) {
                                 case "color":
+                                    if (!TextUtils.isEmpty(value))
                                         try {
                                             int color = Integer.parseInt(value.substring(1), 16) | 0xFF000000;
                                             ssb.setSpan(new ForegroundColorSpan(color), start, ssb.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);