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.

1 files changed, 3 insertions, 0 deletions

diff --git a/command-line-arguments.h b/command-line-arguments.h
index 76ca4dcb..c0228f7c 100644
--- a/command-line-arguments.h
+++ b/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;