Don't try to index empty documentation txt files

If a documentation txt file has no content, parsing the document title
from it in the DocIndexer fails with a NullPointerException:

  Exception in thread "main" java.lang.NullPointerException
    at java.util.regex.Matcher.getTextLength(Matcher.java:1234)
    at java.util.regex.Matcher.reset(Matcher.java:308)
    at java.util.regex.Matcher.<init>(Matcher.java:228)
    at java.util.regex.Pattern.matcher(Pattern.java:1088)
    at DocIndexer.index(DocIndexer.java:117)
    at DocIndexer.invoke(DocIndexer.java:86)
    at DocIndexer.main(DocIndexer.java:163)

Change-Id: Idb4b808847d326ba49c9a2494a95b1cad8eb9406
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

lib/asciidoctor/java/DocIndexer.java

diff --git a/lib/asciidoctor/java/DocIndexer.java b/lib/asciidoctor/java/DocIndexer.java
index 4bcddbb..45cad3d 100644
--- a/lib/asciidoctor/java/DocIndexer.java
+++ b/lib/asciidoctor/java/DocIndexer.java
@@ -104,6 +104,9 @@
     IndexWriter iwriter = new IndexWriter(directory, config);
     for (String inputFile : inputFiles) {
       File file = new File(inputFile);
+      if (file.length() == 0) {
+        continue;
+      }
 
       BufferedReader titleReader = new BufferedReader(
           new InputStreamReader(new FileInputStream(file), "UTF-8"));