vpr_io_place.py fails when no pcf file is provided

[mithro]

chipsalliance/f4pga-examples#236

See https://github.com/SymbiFlow/symbiflow-examples/runs/4655539957?check_suite_focus=true

04:21:58 | make: Entering directory '/root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo'
04:21:58 | mkdir -p /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/build/arty_35
04:21:58 | cd /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/build/arty_35 && symbiflow_synth -t top -v /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/baselitex_arty.v /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/../../third_party/vexriscv-verilog/VexRiscv_Linux.v -d artix7 -p xc7a35tcsg324-1 -x /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/arty.xdc 2>&1 > /dev/null
04:25:19 | cd /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/build/arty_35 && symbiflow_pack -e top.eblif -d xc7a50t_test -s /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/arty.sdc 2>&1 > /dev/null
04:28:56 | cd /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/build/arty_35 && symbiflow_place -e top.eblif -d xc7a50t_test  -n top.net -P xc7a35tcsg324-1 -s /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/arty.sdc 2>&1 > /dev/null
04:28:58 | Traceback (most recent call last):
04:28:58 |   File "/root/opt/symbiflow/xc7/install/share/symbiflow/scripts/prjxray_create_ioplace.py", line 178, in <module>
04:28:58 |     main()
04:28:58 |   File "/root/opt/symbiflow/xc7/install/share/symbiflow/scripts/prjxray_create_ioplace.py", line 169, in main
04:28:58 |     io_place.output_io_place(args.output)
04:28:58 |   File "/root/opt/symbiflow/xc7/install/share/symbiflow/scripts/vpr_io_place.py", line 193, in output_io_place
04:28:58 |     max_name_length = max(len(c.name) for c in self.constraints.values())
04:28:58 | ValueError: max() arg is an empty sequence
04:28:58 | make: *** [/root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/../../common/common.mk:59: /root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo/build/arty_35/top.place] Error 1
04:28:58 | make: Leaving directory '/root/symbiflow-examples/symbiflow-examples/xc7/linux_litex_demo'