Based on the issue description, I understand the problem. The Django `numberformat.py` file has code that checks if `str_number[0] == "-"` to detect negative numbers, but when the number field is null/empty, this causes an `IndexError: string index out of range` because you can't index into an empty string.

The fix is to check if `str_number` is non-empty before checking its first character. Here's the patch:

--- a/django/utils/numberformat.py
+++ b/django/utils/numberformat.py
@@ -18,6 +18,9 @@ def format(number, decimal_sep, decimal_pos=None, grouping=0, thousand_sep='',
     # sign
     sign = ''
     str_number = str(number)
+    if not str_number:
+        return ''
+
     if str_number[0] == '-':
         sign = '-'
         str_number = str_number[1:]