crash when no data to read

[AlanBell]

can be reproduced by reducing the WAIT_AFTER_WAKEUP time on a PMS9003M, was happening occasionally with standard settings

wakeUp
wakeUp got lock
Sending command: 228,1,False,16000
waiting 10.0s
readFrame False False
__read_frame
got no values
Task exception wasn't retrieved
future: coro= <generator object '_read' at 20009660>
Traceback (most recent call last):
File "uasyncio/core.py", line 1, in run_until_complete
File "pms5003.py", line 328, in _read
File "pms5003.py", line 650, in wakeUp
File "pms5003.py", line 628, in _makeResilient
File "pms5003.py", line 192, in wakeUp
File "pms5003.py", line 254, in _sendCommand
File "pms5003.py", line 417, in _read_frame
File "pms5003.py", line 456, in __read_frame
TypeError: object of type 'NoneType' has no len()