Fixed error on empty or CDATA styles. Fixed #455. Fixes #456

plugins/minifyStyles.js

@@ -42,8 +42,8 @@ var minifyCss = function(css, options) {
 exports.fn = function(item, svgoOptions) {
 
     if(item.elem) {
-        if(item.isElem('style')) {
+        if(item.isElem('style') && !item.isEmpty()) {
-            var styleCss = item.content[0].text;
+            var styleCss = item.content[0].text || item.content[0].cdata || [];
             if(styleCss.length > 0) {
                 var styleCssMinified = minifyCss(styleCss, svgoOptions);
                 item.content[0].text = styleCssMinified;

test/plugins/minifyStyles.02.svg

@@ -0,0 +1,19 @@
+<svg id="test" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+    <style>
+        <![CDATA[
+            .st0{ fill:red; padding-top: 1em; padding-right: 1em; padding-bottom: 1em; padding-left: 1em; } @media screen and (max-width: 200px) { .st1 { display: none; } }
+        ]]>
+    </style>
+    <style></style>
+    <rect width="100" height="100" class="st0" style="stroke-width:3; margin-top: 1em; margin-right: 1em; margin-bottom: 1em; margin-left: 1em;"/>
+</svg>
+
+@@@
+
+<svg id="test" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+    <style>
+        .st0{fill:red;padding:1em}@media screen and (max-width:200px){.st1{display:none}}
+    </style>
+    <style/>
+    <rect width="100" height="100" class="st0" style="stroke-width:3;margin:1em"/>
+</svg>