add empty string test case in TestSwapCase in backend/commands/case_test.go
add empty string test case in TestSwapCase in backend/commands/case_test.go

ch1bo/lime-backend,codex8/lime-backend,limetext/backend,bj7/lime-backend,limetext/lime-backend,bj7/lime-backend,bj7/lime-backend,bj7/lime-backend,codex8/lime-backend,limetext/lime-backend,ch1bo/lime-backend,ch1bo/lime-backend,limetext/lime-backend,codex8/lime-backend,ch1bo/lime-backend,codex8/lime-backend,limetext/lime-backend