Fix lexer whitespace bug.

dmc-lib/src/lexer.rs

@@ -27,8 +27,10 @@ impl<'a> Lexer<'a> {
             let maybe_chunk = self.input.get(self.position..);
             if maybe_chunk.is_none() {
                 return None;
-            } else {
-                chunk = maybe_chunk.unwrap();
+            }
+            chunk = maybe_chunk.unwrap();
+            if chunk.is_empty() {
+                return None;
             }
         }
 
@@ -154,4 +156,11 @@ mod tests {
         assert_next(&mut lexer, TokenKind::End, 3);
         assert_eq!(lexer.next(), None);
     }
+    
+    #[test]
+    fn blank_after_last_token_returns_none() {
+        let mut lexer = Lexer::new("fn ");
+        assert_next(&mut lexer, TokenKind::Fn, 2);
+        assert_eq!(lexer.next(), None);
+    }
 }