descriptor.py: fix parsing tr() desc with single leaf TREE #68

Job	Run time
lint	27s
Ledger App builder	1m 36s
bitcoind builder	18m 23s
Dist builder	6h 0m 1s
Non-device tests	27s
Sim builder (trezor-1, trezor-firmware, test/work/trezor-firmware)	1m 50s
Type check	32s
Wine builder	6m 21s
Sim builder (trezor-t, trezor-firmware, test/work/trezor-firmware)	1m 34s
Sim builder (coldcard, coldcard-mpy, test/work/firmware/external/micropython/ports/unix/coldcard-...	8m 55s
Sim builder (bitbox01, mcu, test/work/mcu)	31s
Sim builder (jade, jade, test/work/jade/simulator)	8m 48s
Sim builder (ledger, speculos, test/work/speculos)	1m 34s
Sim builder (keepkey, keepkey-firmware, test/work/keepkey-firmware/bin)	3m 16s
Python ${{ matrix.python-version }} ${{ matrix.device }} ${{ matrix.interface }}	0s
Python ${{ matrix.python-version }} ${{ matrix.device }} ${{ matrix.script.name }}	0s
	6h 54m 15s

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