Relationship between reset pins and SWD functions (ARM Cortex M0)
Reset pin is often not needed, because controller is possible to reset using debug registers.

Only one case when is necessary is connection under reset, this mean that if SWD signals are shared with other functionality on board (forexample SWDIO or SWDCLK is set as output), then only way to access MCU is to put MCU in to reset (at this moment are some debugging registers accessible over SWD) and set HALT/STOP program flag with debug registers over SWD, then leave reset pin and at this moment it is possible to flash MCU.