Fix Matcher.String() with empty label name

When the label name is empty, which can happen now with quoted label
name, it should be quoted when printed as a string again.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

model/labels/matcher.go

@@ -101,7 +101,7 @@ func (m *Matcher) shouldQuoteName() bool {
 		}
 		return true
 	}
-	return false
+	return len(m.Name) == 0
 }
 
 // Matches returns whether the matcher matches the given string value.

promql/parser/printer_test.go

@@ -148,6 +148,13 @@ func TestExprString(t *testing.T) {
 			in:  `{"_0"="1"}`,
 			out: `{_0="1"}`,
 		},
+		{
+			in: `{""="0"}`,
+		},
+		{
+			in:  "{``=\"0\"}",
+			out: `{""="0"}`,
+		},
 	}
 
 	for _, test := range inputs {