fspath: Stop empty strings being a valid path - fixes #4239

Before this change you could use "" as a valid remote, so `rclone lsf
""` would work. This was treated as the current directory.

This is unexpected and creates a footgun for scripting when an empty
variable is passed to rclone by accident.

This fix returns the error "can't use empty string as a path" instead
of allowing it.

fs/fspath/path_test.go

@@ -55,7 +55,7 @@ func TestParse(t *testing.T) {
 		in, wantConfigName, wantFsPath string
 		wantErr                        error
 	}{
-		{"", "", "", nil},
+		{"", "", "", errCantBeEmpty},
 		{":", "", "", errInvalidCharacters},
 		{"::", ":", "", errInvalidCharacters},
 		{":/:", "", "/:", errInvalidCharacters},
@@ -91,7 +91,7 @@ func TestSplit(t *testing.T) {
 		remote, wantParent, wantLeaf string
 		wantErr                      error
 	}{
-		{"", "", "", nil},
+		{"", "", "", errCantBeEmpty},
 
 		{"remote:", "remote:", "", nil},
 		{"remote:potato", "remote:", "potato", nil},