Make literal "{}" expand to itself

This caused major annoyances with e.g. `find -exec`, and it's utterly
useless - "{}" expands to nothing, so why have it at all?

Fixes #1109.

doc_src/faq.hdr

@@ -24,7 +24,6 @@
 - <a href='#faq-titlebar'>I'm seeing weird output before each prompt when using screen. What's wrong?</a>
 - <a href='#faq-greeting'>How do I change the greeting message?</a>
 - <a href='#faq-history'>Why doesn't history substitution ("!$" etc.) work?</a>
-- <a href='#faq-find-braces'>Why do I get a missing argument error with `find ... {}`?</a>
 - <a href='#faq-cd-minus'>How can I use `-` as a shortcut for `cd -`?</a>
 - <a href='#faq-uninstalling'>How do I uninstall fish?</a>
 - <a href='#faq-third-party'>Where can I find extra tools for fish?</a>
@@ -244,21 +243,6 @@ Fish history recall is very simple yet effective:
 
 See <a href='index.html#editor'>documentation</a> for more details about line editing in fish.
 
-<hr>
-\section faq-find-braces Why do I get a missing argument error with `find ... {}`?
-
-Running `find ... -exec ... {}` produces an error:
-
-    find: missing argument to '-exec'
-
-The problem is caused by the empty braces, which are subject to <a href="index.html#expand-brace">brace expansion</a>.
-
-Quote the empty braces to achieve the desired effect:
-
-\fish{cli-dark}
-find ... -exec ... '{{}}'
-\endfish
-
 <hr>
 \section faq-cd-minus How can I use `-` as a shortcut for `cd -`?
 

doc_src/index.hdr.in

@@ -506,6 +506,14 @@ mv *.{c,h} src/
 # Moves all files with the suffix '.c' or '.h' to the subdirectory src.
 \endfish
 
+A literal "{}" will not be used as a brace expansion:
+
+\fish
+echo foo-{}
+# Outputs foo-{}
+
+echo foo-{$undefinedvar}
+# Output is an empty line - see <a href="#cartesian-product">the cartesian product section</a>
 
 \subsection expand-variable Variable expansion