Make selecting inner object fail on empty objects

Trying to select the inside of say '{}' was previously selecting the
closing character, this is not very useful or predictible. Change
the behaviour to drop that selection which is more consistent with
the rest of Kakoune's  editing language.

Closes #5422

Author: mawww

src/selectors.cc

@@ -370,7 +370,7 @@ find_surrounding(const Container& container, Iterator pos,
             return {};
     }
     if (first > last)
-        last = first;
+        return {};
 
     return std::pair<Iterator, Iterator>{first, last};
 }
@@ -1230,7 +1230,9 @@ UnitTest test_find_surrounding{[]()
                                     Regex{"\\Q" + opening, RegexCompileFlags::Backward},
                                     Regex{"\\Q" + closing, RegexCompileFlags::Backward},
                                     flags, level);
-        kak_assert(res);
+        kak_assert((not res) == expected.empty());
+        if (not res)
+            return;
         auto min = std::min(res->first, res->second),
              max = std::max(res->first, res->second);
         kak_assert(StringView{min, max+1} == expected);
@@ -1240,7 +1242,7 @@ UnitTest test_find_surrounding{[]()
     check_equal(s.begin() + 13, '[', ']', ObjectFlags::ToBegin | ObjectFlags::ToEnd | ObjectFlags::Inner, 0, "bar { baz[] }");
     check_equal(s.begin() + 5, '[', ']', ObjectFlags::ToBegin | ObjectFlags::ToEnd, 0, "[bar { baz[] }]");
     check_equal(s.begin() + 10, '{', '}', ObjectFlags::ToBegin | ObjectFlags::ToEnd, 0, "{ baz[] }");
-    check_equal(s.begin() + 16, '[', ']', ObjectFlags::ToBegin | ObjectFlags::ToEnd | ObjectFlags::Inner, 0, "]");
+    check_equal(s.begin() + 16, '[', ']', ObjectFlags::ToBegin | ObjectFlags::ToEnd | ObjectFlags::Inner, 0, "");
     check_equal(s.begin() + 18, '[', ']', ObjectFlags::ToBegin | ObjectFlags::ToEnd, 0, "[bar { baz[] }]");
     check_equal(s.begin() + 6, '[', ']', ObjectFlags::ToBegin, 0, "[b");
 

test/normal/object/inner/empty-braces/cmd

@@ -0,0 +1 @@
+<a-i>{

test/normal/object/inner/empty-braces/error

@@ -0,0 +1 @@
+'exec': no selections remaining

test/normal/object/inner/empty-braces/in

@@ -0,0 +1 @@
+%({)} {%({)}} {%(}) {{%(})}

test/normal/object/inner/empty-double_quote/cmd

@@ -0,0 +1 @@
+<a-i>{

test/normal/object/inner/empty-double_quote/error

@@ -0,0 +1 @@
+'exec': no selections remaining

test/normal/object/inner/empty-double_quote/in

@@ -0,0 +1 @@
+""