[join-] fix putValue for merge rows absent in any source sheet

[midichef]

When editing cells in a MergeColumn, an error will occur if the row is absent from any of the source sheets. putValue() will attempt to set the value on the source sheet that does not have the row, causing an error:

  File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/features/join.py", line 169, in putValue
    c.setValue(row[vs], value)
...
  File "/home/midichef/.venv/lib/python3.12/site-packages/visidata/utils.py", line 73, in setitem
    r[i] = v
TypeError: 'NoneType' object does not support item assignment

To reproduce this, unzip merge_rows.zip, and do vd -p test.vdj
Then edit cells in the source_sheet column. The cell with source_sheet of b will have the error after you try to edit it with e and enter any value. The cell for the a_only row will also give the error, though the edit will succeed.

This PR adds a check similar to the one in JoinKeyColumn:

visidata/visidata/features/join.py

Lines 148 to 149 in efbe84e

	if row[c.sheet] is not None:
	c.setValues([row[c.sheet]], value)