Fix default errors for bates_number()

[BryceStevenWilley]

If any of the font_size, offset_horizontal, and offset_vertical params aren't given, then bates_number will error, because the default values for those params are given as None, which do not parse into floats on the bates.py side.

This patch changes the defaults in util.py to match the defaults in marisol.py: 15 for the offsets and 10 for the font size.

Also changes how the values are retrieved from kwargs, to prevent people from breaking the function by passing in Nones.

Introduced in #898.

Smallest program that'll fair in 1.9.2.

event: done
question: |
  Your document
subquestion: |
  ${ test_document }
---
mandatory: True
code: |
  test_document.bates_number()
  done
---
attachments:
  - variable name: test_document
    filename: test_document
    valid formats: [pdf]
    content: "[BOLDCENTER] A Test Document [BR]"

Fails with:

bates_number: failure during processing; return value 2 after /usr/share/docassemble/local3.12/bin/python -m docassemble.base.bates --prefix MI- --digits 3 --start 0 --area BOTTOM_LEFT --font-size 5 --offset-horizontal None --offset-vertical 250 /usr/share/docassemble/files/000/000/000/55c/file.pdf