cli: Prevent weird `rad issue open` behavior

Verify special YAML strings '~' and 'null' are filtered out when
processing the input from the editor.

Serde replaces empty strings with their values causing an empty title to
become '~'.  Best to avoid them entirely and be transparent about it.

The solution is not meant to be long term.  This editor format for
getting issue data will be replaced in the near term.

radicle-cli/src/commands/issue.rs

@@ -507,10 +507,12 @@ fn prompt_issue(
         serde_yaml::from_str(&meta).context("failed to parse yaml front-matter")?;
 
     meta.title = meta.title.trim().to_string();
-    if meta.title.is_empty() {
+    if meta.title.is_empty() || meta.title == "~" || meta.title == "null" {
+        // '~' and 'null' are YAML's string values for null and unexpectedly replace empty fields
+        // for String.
         return Err(io::Error::new(
             io::ErrorKind::InvalidInput,
-            "an issue title must be provided",
+            "an issue title must be provided and may not be '~' or 'null'",
         )
         .into());
     }