Initialise color to fix translation to C#

src/main/java/org/zwobble/mammoth/internal/styles/parsing/DocumentMatcherParser.java

@@ -116,7 +116,7 @@ private static boolean parseListType(TokenIterator<TokenType> tokens) {
     }
 
     private static HighlightMatcher parseHighlightMatcher(TokenIterator<TokenType> tokens) {
-        Optional<String> color;
+        Optional<String> color = Optional.empty();
 
         if (tokens.trySkip(TokenType.SYMBOL, "[")) {
             tokens.skip(TokenType.IDENTIFIER, "color");