ePWM avoid using CMP = 0

please see F2837xD reference manual 14.6.5，it reads：
Use up-down-count mode to generate a symmetric PWM:
• If you load CMPA/CMPB on zero, then use CMPA/CMPB values greater
than or equal to 1.
• If you load CMPA/CMPB on period, then use CMPA/CMPB values less than
or equal to TBPRD-1.
This means there will always be a pulse of at least one TBCLK cycle in a
PWM period which, when very short, tend to be ignored by the system.

then the code configures as
(*ePWM[n]).CMPCTL.bit.SHDWAMODE = CC_SHADOW;
(*ePWM[n]).CMPCTL.bit.LOADAMODE = CC_CTR_ZERO;
(*ePWM[n]).CMPCTL.bit.SHDWBMODE = CC_SHADOW;
(*ePWM[n]).CMPCTL.bit.LOADBMODE = CC_CTR_ZERO;
it load CMPA and CMPB on zero，so (*ePWM[n]).CMPA.bit.CMPA = 1
it does not make difference between (*ePWM[n]).CMPB.bit.CMPB = （period/2 - 1） and （period/2） because the 1 count time is very short and can be ignored