fix(config): ignore empty string patterns

An empty string pattern is a mistake - there is no reason to do that. It can however happen (for instance when you generate the config file).

Before this fix, Karma would try to watch this empty string pattern, which would result in watching the entire `basePath` directory.

Now, the pattern will be ignored and a warning showed.

lib/config.js

@@ -23,18 +23,19 @@ var UrlPattern = function(url) {
 
 
 var createPatternObject = function(pattern) {
-  if (helper.isString(pattern)) {
+  if (pattern && helper.isString(pattern)) {
     return helper.isUrlAbsolute(pattern) ? new UrlPattern(pattern) : new Pattern(pattern);
   }
 
   if (helper.isObject(pattern)) {
-    if (!helper.isDefined(pattern.pattern)) {
-      log.warn('Invalid pattern %s!\n\tObject is missing "pattern" property".', pattern);
-    }
-
-    return helper.isUrlAbsolute(pattern.pattern) ?
+    if (pattern.pattern && helper.isString(pattern.pattern)) {
+      return helper.isUrlAbsolute(pattern.pattern) ?
         new UrlPattern(pattern.pattern) :
         new Pattern(pattern.pattern, pattern.served, pattern.included, pattern.watched);
+    }
+
+    log.warn('Invalid pattern %s!\n\tObject is missing "pattern" property.', pattern);
+    return new Pattern(null, false, false, false);
   }
 
   log.warn('Invalid pattern %s!\n\tExpected string or object with "pattern" property.', pattern);