Issue #7 Ignore keywords in the trie that are null or empty

src/main/java/org/ahocorasick/trie/Trie.java

@@ -47,6 +47,9 @@ public class Trie {
     }
 
     public void addKeyword(String keyword) {
+        if (keyword == null || keyword.length() == 0) {
+            return;
+        }
         State currentState = this.rootState;
         for (Character character : keyword.toCharArray()) {
             currentState = currentState.addState(character);

src/test/java/org/ahocorasick/trie/TrieTest.java

@@ -193,6 +193,19 @@ public class TrieTest {
         assertEquals(5, tokens.size());
     }
 
+    @Test
+    public void zeroLengthTestBug7InGithubReportedByXCurry() {
+        Trie trie = new Trie().removeOverlaps().onlyWholeWords().caseInsensitive();
+        trie.addKeyword("");
+
+        Collection<Token> tokens = trie.tokenize("Try a natural lip and subtle bronzer to keep all the focus on those big bright eyes with NARS Eyeshadow Duo in Rated R And the winner is... Boots No7 Advanced Renewal Anti-ageing Glycolic Peel Kit ($25 amazon.com) won most-appealing peel.");
+        for (Token token : tokens) {
+            if (!token.isMatch()) {
+                System.out.println("token: " + token.getFragment());
+            }
+        }
+    }
+
     private void checkEmit(Emit next, int expectedStart, int expectedEnd, String expectedKeyword) {
         assertEquals(expectedStart, next.getStart());
         assertEquals(expectedEnd, next.getEnd());