Avoid falling back to non-context completions

If there are no context completions (e.g. if there are no namespaces
starting with the prefix), prior to this commit, Tutkain would suggest
non-context completions instead.

clojure/repl/completions.repl

@@ -437,3 +437,17 @@
   (spec/and
     (prefixed-candidates ::specs/class-completions "j")
     (partial not-any? (comp #{:snippet} :completion-type))))
+
+(intern (the-ns 'user) 'NONE)
+
+;; do not propose non-context completions if there are no context completions
+(completions
+  {:prefix "NONE"
+   :ns "user"
+   :enclosing-sexp "(require '[])"
+   :start-line 1
+   :start-column 1
+   :line 1
+   :column 12})
+
+(xr/check! empty?)

clojure/src/tutkain/completions.cljc

@@ -631,7 +631,7 @@
                              (require-completions loc prefix))
       clojure.core/import (let [loc (find-loc form line column)]
                             (import-completions loc prefix))
-      [])))
+      ::none)))
 
 (defn ^:private find-completions
   [{:keys [file ns prefix enclosing-sexp start-line start-column line column] :as message}]
@@ -643,9 +643,10 @@
                     {:features #{:clj :t.a.jvm} :read-cond :allow}
                     start-line
                     start-column)))]
-    (or
-      (not-empty (context-completions (peek forms) prefix line column))
-      (into (local-completions forms message) (candidates prefix ns)))))
+    (let [context-completions (context-completions (peek forms) prefix line column)]
+      (if (not (identical? ::none context-completions))
+        context-completions
+        (into (local-completions forms message) (candidates prefix ns))))))
 
 (defmethod completions :default
   [message]