cli: allow empty strings for notmuch insert --folder argument

Now that it's easy to add argument specific modifiers in opt
descriptions, add a new .allow_empty field to allow empty strings for
individual string arguments while retaining strict checks
elsewhere. Use this for notmuch insert --folder, where the empty
string means top level folder.

command-line-arguments.c

@@ -81,7 +81,7 @@ _process_string_arg (const notmuch_opt_desc_t *arg_desc, char next, const char *
 	fprintf (stderr, "Option \"%s\" needs a string argument.\n", arg_desc->name);
 	return false;
     }
-    if (arg_str[0] == '\0') {
+    if (arg_str[0] == '\0' && ! arg_desc->allow_empty) {
 	fprintf (stderr, "String argument for option \"%s\" must be non-empty.\n", arg_desc->name);
 	return false;
     }

command-line-arguments.h

@@ -32,6 +32,9 @@ typedef struct notmuch_opt_desc {
     /* Optional, if non-NULL, set to true if the option is present. */
     bool *present;
 
+    /* Optional, allow empty strings for opt_string. */
+    bool allow_empty;
+
     /* Must be set for opt_keyword and opt_flags. */
     const struct notmuch_keyword *keywords;
 } notmuch_opt_desc_t;

doc/man1/notmuch-insert.rst

@@ -34,7 +34,8 @@ Supported options for **insert** include
     ``--folder=<``\ folder\ **>**
         Deliver the message to the specified folder, relative to the
         top-level directory given by the value of **database.path**. The
-        default is to deliver to the top-level directory.
+        default is the empty string, which means delivering to the
+        top-level directory.
 
     ``--create-folder``
         Try to create the folder named by the ``--folder`` option, if it

notmuch-insert.c

@@ -464,7 +464,7 @@ notmuch_insert_command (notmuch_config_t *config, int argc, char *argv[])
     unsigned int i;
 
     notmuch_opt_desc_t options[] = {
-	{ .opt_string = &folder, .name = "folder" },
+	{ .opt_string = &folder, .name = "folder", .allow_empty = true },
 	{ .opt_bool = &create_folder, .name = "create-folder" },
 	{ .opt_bool = &keep, .name = "keep" },
 	{ .opt_bool =  &no_hooks, .name = "no-hooks" },