Disallow empty value in list.

$ echo "x [a,  , b]" | blaze-bin/third_party/txtpbfmt/fmt
parser.Format for path <stdin> with content "x [a,  , b]\n" returned err Empty value in list.
F0428 14:10:56.453731  680717 fmt.go:103] 1 error(s) encountered during execution

Note that this doesn't prevent empty strings:

$ echo "x [a, '' , b]" | blaze-bin/third_party/txtpbfmt/fmt
x [a, "", b]

PiperOrigin-RevId: 445142675

parser/parser.go

@@ -714,7 +714,10 @@ func (p *parser) parse(isRoot bool) (result []*ast.Node, endPos ast.Position, er
 						return nil, ast.Position{}, err
 					}
 					if len(vals) != 1 {
-						return nil, ast.Position{}, fmt.Errorf("multiple-string value not supported (%v). Please add comma explcitily, see http://b/162070952", vals)
+						return nil, ast.Position{}, fmt.Errorf("multiple-string value not supported (%v). Please add comma explicitly, see http://b/162070952", vals)
+					}
+					if len(vals[0].Value) == 0 {
+						return nil, ast.Position{}, fmt.Errorf("empty value in list")
 					}
 					vals[0].PreComments = append(vals[0].PreComments, preComments...)
 

parser/parser_test.go

@@ -26,6 +26,8 @@ func TestError(t *testing.T) {
 	}, {
 		in: `name: "string with literal new line
 "`, err: "new line",
+	}, {
+		in: `list_with_empty_list: [a,,b]`, err: "empty value in list",
 	}}
 	for _, input := range inputs {
 		out, err := Format([]byte(input.in))