avoid calling CssStyleFormatter.mergeStyleAttributes if parentStyle or childStyle are null

Author: mzorz

aztec/src/main/kotlin/org/wordpress/aztec/plugins/CssUnderlinePlugin.kt

@@ -41,7 +41,9 @@ class CssUnderlinePlugin : ISpanPostprocessor, ISpanPreprocessor {
                     if (hiddenSpan.TAG == SPAN_TAG) {
                         val parentStyle = hiddenSpan.attributes.getValue(CssStyleFormatter.STYLE_ATTRIBUTE)
                         val childStyle = calypsoUnderlineSpan.attributes.getValue(CssStyleFormatter.STYLE_ATTRIBUTE)
-                        hiddenSpan.attributes.setValue(CssStyleFormatter.STYLE_ATTRIBUTE, CssStyleFormatter.mergeStyleAttributes(parentStyle, childStyle))
+                        if (parentStyle != null && childStyle != null) {
+                            hiddenSpan.attributes.setValue(CssStyleFormatter.STYLE_ATTRIBUTE, CssStyleFormatter.mergeStyleAttributes(parentStyle, childStyle))
+                        }
 
                         // remove the extra child span
                         spannable.removeSpan(calypsoUnderlineSpan)