Giving a field an empty label breaks `BoundField.label_tag` (IndexError)

[Baptiste Mispelon]

The following code shows the regression:

from django import forms

class F(forms.Form):
    foo = forms.CharField(label='')

print(F()['foo'].label_tag())

On django 1.5, this prints <label for="id_foo"></label>. On master, it raises an IndexError.

This is regression was introduced by 584bd14dcfdee9585fec7794d53ce120ea73d0bc.
Previously, calling the label_tag on a field with an empty label would

[Baptiste Mispelon <bmispelon@…>]

In 3128f3d38d1db0bc01da9a4bf4be81119079d73a:

Fixed #20618 -- Fixed regression in BoundField.label_tag.