in_dxf: MINSERT column/row count default to 1 when the group is omitted

[tuxiasumari]

Symptom

A single-column MINSERT (71=2, no 70 group) round-trips as 70=0: DXF omits INSERT groups 70/71 when the count is 1 — ezdxf and AutoCAD both do — but the importer materialized the missing group as 0, and dwg2dxf then writes the explicit 0 back out.

Fix

Apply the documented defaults when the MINSERT completes, next to the existing 3DFACE default in the same chain. (Setting them earlier, at the AcDbMInsertBlock upgrade, does not work: the generic field matcher refuses to overwrite an already-set field when the real 70/71 group arrives.)

import ezdxf
d = ezdxf.new("R2000")
blk = d.blocks.new("B1"); blk.add_circle((0, 0), 1)
ref = d.modelspace().add_blockref("B1", (0, 0))
ref.grid(size=(2, 1), spacing=(10, 10))   # 2 rows, 1 column
d.saveas("minsert.dxf")

make check green; 1657-file real-DWG corpus unchanged.