shared: add nm_str_is_empty() helper

We have nm_str_not_empty() which is the inverse of that. The purpose
of nm_str_not_empty() is to normalize a string to either return
%NULL or a non-empty string, like

   const char *
   get_name (Object *obj)
   {
        return nm_str_not_empty (obj->name);
   }

Sometimes, we however want to check whether a string is not empty.
So, we previously had two choices:

1) use a temporary variable:

     const char *tmp;

     tmp = get_string ();
     if (tmp && tmp[0])
        ...

The problem with this variant is that it's more verbose (by requiring a
temporary variable). Another downside is that there are multiple ways
how to check for an empty string (!tmp[0], tmp[0] == '\0', !strlen (tmp),
strlen (tmp) == 0), and sure enough they are all in use.

2) use !nm_str_not_empty(). But this double negation looks really odd
and confusing.

Add nm_str_is_empty() instead.

shared/nm-glib-aux/nm-macros-internal.h

@@ -989,16 +989,23 @@ nm_gstring_add_space_delimiter (GString *str)
 	return str;
 }
 
+static inline gboolean
+nm_str_is_empty (const char *str)
+{
+	/* %NULL is also accepted, and also "empty". */
+	return !str || !str[0];
+}
+
 static inline const char *
 nm_str_not_empty (const char *str)
 {
-	return str && str[0] ? str : NULL;
+	return !nm_str_is_empty (str) ? str : NULL;
 }
 
 static inline char *
 nm_strdup_not_empty (const char *str)
 {
-	return str && str[0] ? g_strdup (str) : NULL;
+	return !nm_str_is_empty (str) ? g_strdup (str) : NULL;
 }
 
 static inline char *