proc_clean: Fix empty case removal conditions.
proc_clean: Fix empty case removal conditions.

Fixes #2639.

cliffordwolf/yosys,antmicro/yosys,antmicro/yosys,antmicro/yosys,antmicro/yosys,antmicro/yosys,antmicro/yosys,antmicro/yosys,cliffordwolf/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys,YosysHQ/yosys,YosysHQ/yosys,antmicro/yosys,cliffordwolf/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,cliffordwolf/yosys