fix(eval): checking for a non-empty string is too strict (#15987)

Cherry-pick check_for_nonempty_string() from patch vim-8.2.2133 and
apply it on the bases of https://github.com/neovim/neovim/pull/13489

https://github.com/vim/vim/commit/2a9d5d386bea8455b37c1accebc45683ec51d6fb

test/functional/vimscript/executable_spec.lua

@@ -18,7 +18,7 @@ describe('executable()', function()
   end)
 
   it('fails for invalid values', function()
-    for _, input in ipairs({'""', 'v:null', 'v:true', 'v:false', '{}', '[]'}) do
+    for _, input in ipairs({'v:null', 'v:true', 'v:false', '{}', '[]'}) do
       eq('Vim(call):E928: String required', exc_exec('call executable('..input..')'))
     end
     command('let $PATH = fnamemodify("./test/functional/fixtures/bin", ":p")')
@@ -27,6 +27,10 @@ describe('executable()', function()
     end
   end)
 
+  it('returns 0 for empty strings', function()
+    eq(0, call('executable', '""'))
+  end)
+
   it('returns 0 for non-existent files', function()
     eq(0, call('executable', 'no_such_file_exists_209ufq23f'))
   end)