glib-aux: fix nicks for zero flag in nm_utils_enum_to_str()

nm_utils_enum_to_str() can print flags, that is, combinations of
powers of two integers.

It also supports nicks, for certain flags.

When we have a nick for value zero, then that requires special
handling. Otherwise, that zero nick will always show up in the
string representation, although, it should only be used if the
enum value is exactly zero.

(cherry picked from commit eec9efd989)

src/libnm-core-impl/tests/test-general.c

@@ -9123,6 +9123,10 @@ test_nm_utils_enum(void)
             .nick  = "nick-5",
             .value = 5,
         },
+        {
+            .nick  = "nick-0",
+            .value = 0,
+        },
         {
             .nick  = "nick-red",
             .value = NM_TEST_GENERAL_COLOR_FLAGS_RED,
@@ -9170,6 +9174,11 @@ test_nm_utils_enum(void)
                                        "nick-5, green",
                                        color_value_infos);
 
+    _test_nm_utils_enum_to_str_do_full(color_flags,
+                                       0,
+                                       "nick-0",
+                                       color_value_infos);
+
     _test_nm_utils_enum_from_str_do(bool_enum, "", FALSE, 0, NULL);
     _test_nm_utils_enum_from_str_do(bool_enum, " ", FALSE, 0, NULL);
     _test_nm_utils_enum_from_str_do(bool_enum, "invalid", FALSE, 0, "invalid");

src/libnm-glib-aux/nm-enum-utils.c

@@ -136,7 +136,8 @@ _nm_utils_enum_to_str_full(GType                       type,
         else
             return g_strdup(enum_value->value_nick);
     } else if (G_IS_FLAGS_CLASS(klass)) {
-        unsigned     uvalue = (unsigned) value;
+        unsigned     uvalue          = (unsigned) value;
+        gboolean     uvalue_was_zero = (uvalue == 0);
         GFlagsValue *flags_value;
         NMStrBuf     strbuf;
 
@@ -147,6 +148,9 @@ _nm_utils_enum_to_str_full(GType                       type,
         for (; value_infos && value_infos->nick; value_infos++) {
             nm_assert(_enum_is_valid_flags_nick(value_infos->nick));
 
+            if (value_infos->value == 0 && !uvalue_was_zero)
+                continue;
+
             if (uvalue == 0) {
                 if (value_infos->value != 0)
                     continue;