BUG: sniffing a csv raises with only a header

Closes #7773. Validation Test.

Author: gfyoung <gfyoung17@gmail.com>

Closes #12914 from gfyoung/sniffer-header-only and squashes the following commits:

e0cb4b0 [gfyoung] TST: Verify that #7773 is fixed

Author: gfyoung

pandas/io/tests/test_parsers.py

@@ -2827,6 +2827,16 @@ def test_read_with_bad_header(self):
             s = StringIO(',,')
             self.read_csv(s, header=[10])
 
+    def test_read_only_header_no_rows(self):
+        # See gh-7773
+        expected = DataFrame(columns=['a', 'b', 'c'])
+
+        df = self.read_csv(StringIO('a,b,c'))
+        tm.assert_frame_equal(df, expected)
+
+        df = self.read_csv(StringIO('a,b,c'), index_col=False)
+        tm.assert_frame_equal(df, expected)
+
 
 class CompressionTests(object):
     def test_zip(self):