Handle empty documents.

Fixes #31

src/lib.rs

@@ -229,7 +229,7 @@ fn add_toc(content: &str, cfg: &Config) -> Result<String> {
         log::trace!("content_after_toc={:?}", content_after_toc);
         // Multiline markers might have consumed trailing newlines,
         // we ensure there's always one before the content.
-        let extra = if content_after_toc.as_bytes()[0] == b'\n' {
+        let extra = if content_after_toc.is_empty() || content_after_toc.as_bytes()[0] == b'\n' {
             ""
         } else {
             "\n"

tests/empty_document.in.md

@@ -0,0 +1,3 @@
+# Chapter 1
+
+<!-- toc -->

tests/empty_document.out.md

@@ -0,0 +1,2 @@
+# Chapter 1
+

tests/it.rs

@@ -157,3 +157,10 @@ fn backslash_escapes() {
     // Backslash-escaped elements should still be escaped.
     assert_toc!("backslash_escapes");
 }
+
+#[test]
+fn empty_document() {
+    // Regression test #31
+    // Empty documents should not fail
+    assert_toc!("empty_document");
+}