posts: fix exception when searching for "-" or "~".

app/logical/post_query_builder.rb

@@ -1038,7 +1038,9 @@ class PostQueryBuilder
       def parse_tag(tag, output)
         operator, tag = parse_tag_operator(tag)
 
-        if tag.include?("*")
+        if tag.blank?
+          # XXX ignore "-", "~" operators without a tag.
+        elsif tag.include?("*")
           tags = Tag.wildcard_matches(tag)
 
           if operator == "-"

test/unit/post_test.rb

@@ -1962,6 +1962,13 @@ class PostTest < ActiveSupport::TestCase
       assert_tag_match([post2], "-*c -a*a")
     end
 
+    should "ignore invalid operator syntax" do
+      assert_nothing_raised do
+        assert_tag_match([], "-")
+        assert_tag_match([], "~")
+      end
+    end
+
     should "return posts for the id:<N> metatag" do
       posts = FactoryBot.create_list(:post, 3)