Force the wrap before a wrapped row that starts with an empty cell

[daviwil]

Fixes #6230

_rowEnd decides whether the next row's wrap happens on its own, which needs a character to be printed. The check read

nextRowFirstChar.getChars() &&
  isNextRowFirstCharDoubleWidth ? this._nullCellCount <= 1 : this._nullCellCount <= 0

which parses as (chars && wide) ? a : b. When the wrapped row starts with an empty cell and the row above is full, it fell through to this._nullCellCount <= 0 and passed. The serializer then wrote the empty cell as CSI 1 C, which doesn't wrap, so the replay printed the rest of the row over the last cell of the row above.

The fix adds the parentheses the comment above it describes, so an empty first cell always takes the forced wrap path (-, CSI 1 D, CSI 1 X). Cases with a printed first cell are unchanged.

For the repro in the issue, the frame goes from abcdefghij\x1b[1CYZ\x1b[3D to abcdefghij-\x1b[1D\x1b[1X\x1b[1CYZ\x1b[3D, and the replay matches the source, wrap flag included.

Testing:

• New integration test handle wrapped row starting with empty cell, using testNormalScreenEqual. It fails on master and passes with the fix.
• addon-serialize integration suite on Chromium: 41 passed. I didn't run Firefox or WebKit locally.
• npm run test-unit: 2403 passing. npm run lint-changes is clean.
• Checked by hand against headless with a few nearby cases: two empty cells, an empty cell with a background, a wide character after the empty cells, a wide character at the edge, and a plain wrap. All round-trip. Master fails on the first and third of those.

This is independent of the fix for #6231 in the same function. The two branches merge cleanly in either order.