Amend bc73a85778ffb1432bdc510d9a710e7394f42163

Missing return in `specialCharKindToXMLEntity`. Previously, the
functions that were merged into `specialCharKindToXMLEntity` did not
return any kind of error in case an unknown special character is met
(enumerated value). This behaviour is preserved.

src/insets/InsetSpecialChar.cpp

@@ -565,6 +565,8 @@ string specialCharKindToXMLEntity(InsetSpecialChar::Kind kind) {
 		return "LaTeX2ε";
 	case InsetSpecialChar::Kind::PHRASE_LATEX:
 		return "LaTeX";
+	default:
+		return "";
 	}
 }
 }