Fix: longest_substr([]) -> ''

Previously the algorithm used to find the longest common starting substring
unintentionally raised an error when it was called on an empty list.

IPython/utils/tests/test_text.py

@@ -149,6 +149,9 b' def test_long_substr2():'
     data = ['abc', 'abd', 'abf', 'ab']
     nt.assert_equal(text.long_substr(data), 'ab')
 
+def test_long_substr_empty():
+    data = []
+    nt.assert_equal(text.long_substr(data), '')
 
 def test_strip_email():
     src = """\

IPython/utils/text.py

@@ -553,7 +553,7 b' def long_substr(data):'
             for j in range(len(data[0])-i+1):
                 if j > len(substr) and all(data[0][i:i+j] in x for x in data):
                     substr = data[0][i:i+j]
-    else:
+    elif len(data) == 1:
         substr = data[0]
     return substr