Fix filldown RuntimeError on header-only tables

filldown eagerly reads the first data row to seed its fill values, so a
table with a header but no data rows raised
"RuntimeError: generator raised StopIteration" (the StopIteration from
next() escaping the generator under PEP 479). fillright and fillleft
already handle that input by returning the header unchanged; guard the
seed read so filldown behaves the same.

Author: sarathfrancis90

petl/test/transform/test_fills.py

@@ -60,6 +60,19 @@ def test_filldown_headerless():
     ieq(expect, actual)
 
 
+def test_filldown_header_only():
+    table = (('foo', 'bar', 'baz'),)
+    expect = (('foo', 'bar', 'baz'),)
+
+    actual = filldown(table)
+    ieq(expect, actual)
+    ieq(expect, actual)
+
+    actual = filldown(table, 'bar')
+    ieq(expect, actual)
+    ieq(expect, actual)
+
+
 def test_fillright():
 
     table = (('foo', 'bar', 'baz'),

petl/transform/fills.py

@@ -112,7 +112,10 @@ def iterfilldown(table, fillfields, missing):
     if not fillfields:  # fill down all fields
         fillfields = hdr
     fillindices = asindices(hdr, fillfields)
-    fill = list(next(it))  # fill values
+    try:
+        fill = list(next(it))  # fill values
+    except StopIteration:
+        return
     yield tuple(fill)
     for row in it:
         outrow = list(row)