Fix code_to_line when called with empty code

Previously, it was referring to line before it was assigned. Noticed
when testing the R kernel.

IPython/kernel/adapter.py

@@ -15,6 +15,8 b' def code_to_line(code, cursor_pos):'
     
     For adapting ``complete_`` and ``object_info_request``.
     """
+    if not code:
+        return "", 0
     for line in code.splitlines(True):
         n = len(line)
         if cursor_pos > n:

IPython/kernel/tests/test_adapter.py

@@ -8,7 +8,7 b' import json'
 from unittest import TestCase
 import nose.tools as nt
 
-from IPython.kernel.adapter import adapt, V4toV5, V5toV4
+from IPython.kernel.adapter import adapt, V4toV5, V5toV4, code_to_line
 from IPython.kernel.zmq.session import Session
 
 
@@ -20,6 +20,10 b' def test_default_version():'
     adapted = adapt(original)
     nt.assert_equal(adapted['header']['version'], V4toV5.version)
 
+def test_code_to_line_no_code():
+    line, pos = code_to_line("", 0)
+    nt.assert_equal(line, "")
+    nt.assert_equal(pos, 0)
 
 class AdapterTest(TestCase):