Hancle empty lines properly.

(Whitespace was handled, a really empty line was not.)

Address bollwyvl#22.

hy_kernel/hy_kernel.py

@@ -90,7 +90,7 @@ def do_execute(self, code, *args, **kwargs):
                     if line[:2] == "%%":
                         # cell magic
                         cell.append(line)
-                    elif line[:1] in "!%":
+                    elif len(line) > 0 and line[0] in "!%":
                         # line magic
                         if chunk:
                             cell.append(self.compile_chunk(chunk))