do not add none trackitems to the multitrack #1667

Job	Run time
cpython (3.8)	25s
doctest-lxml (3.12)	48s
cpython-lxml (3.8)	3m 22s
static-tests (3.12)	40s
cpython (3.9)	20s
cpython-lxml (3.9)	3m 13s
cpython (3.10)	15s
cpython-lxml (3.10)	3m 53s
Build & inspect our package.	23s
pypy (pypy-3.8)	54s
cpython-lxml (3.11)	3m 48s
cpython (3.11)	21s
pypy (pypy-3.9)	47s
cpython-lxml (3.12)	4m 50s
cpython (3.12)	34s
pypy (pypy-3.10)	43s
cpython-lxml (3.13)	5m 0s
cpython (3.13)	24s
cpython (3.14-dev)	32s
Test install on TestPyPI	16s
Publish to PyPI on push to main	0s
	31m 28s

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