fix(logql): Fix panic in json parsing when using invalid array index

This PR fixes a panic that occurs with the following json parser
expression:

```
| json keys[""]
```

when the log line is the following:

```json
{"keys": ["a", "b", "c"]}
```

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

pkg/logql/log/parser_test.go

@@ -368,6 +368,26 @@ func TestJSONExpressionParser(t *testing.T) {
 			labels.FromStrings("param", "1"),
 			NoParserHints(),
 		},
+		{
+			"object element not present",
+			testLine,
+			[]LabelExtractionExpr{
+				NewLabelExtractionExpr("undefined", `pod[""]`),
+			},
+			labels.EmptyLabels(),
+			labels.FromStrings("undefined", ""),
+			NoParserHints(),
+		},
+		{
+			"accessing invalid array index",
+			testLine,
+			[]LabelExtractionExpr{
+				NewLabelExtractionExpr("param", `pod.deployment.params[""]`),
+			},
+			labels.EmptyLabels(),
+			labels.FromStrings("param", ""),
+			NoParserHints(),
+		},
 		{
 			"array string element",
 			testLine,