1.2.0: _block_width treats a zero page margin as 1 inch, so tables are too narrow #1638

Since 1.2.0, Document._block_width falls back to 1 inch when a margin is falsy:

left_margin = section.left_margin or Inches(1)
right_margin = section.right_margin or Inches(1)

A margin explicitly set to 0 is falsy, so it counts as 1 inch. add_table() then computes the grid (w:tblGrid/w:gridCol) one inch narrower per zero margin. In 1.1.2 the width was page_width - left_margin - right_margin, and the table spanned the full page.

import docx
from docx.shared import Mm

doc = docx.Document()
section = doc.sections[-1]
section.page_width, section.page_height = Mm(210), Mm(297)
section.left_margin = section.right_margin = 0
table = doc.add_table(rows=1, cols=2)
print(docx.__version__, [col.width for col in table.columns], "page", section.page_width)

1.1.2 [3780155, 3780155] page 7560310
1.2.0 [2865755, 2865755] page 7560310

Word lays the table out from the cell widths and looks fine, but LibreOffice follows the grid and cuts text off. We hit it through pdf2docx, which copies the PDF's margins (often 0) into the section.

Suggested fix: fall back only when the value is missing, e.g. Inches(1) if section.left_margin is None else section.left_margin (same for right_margin and page_width).