drivers: serial: nrfx_uarte2: Fix error event not being sent

When RX length was 0 and error was reported then UART_RX_STOPPED event
was not sent. Fixing the condition check to ensure that UART_RX_STOPPED
is always generated.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>