patch 8.2.4896: expression in command block does not look after NL

Problem:    Expression in command block does not look after NL when command is
            typed.
Solution:   Skip over NL also when not in a script. (closes #10358)

src/testdir/test_usercommands.vim

@@ -705,6 +705,11 @@ func Test_usercmd_with_block()
   END
   call v9.CheckScriptSuccess(lines)
   call assert_equal('true', g:result)
+  unlet g:result
+
+  call feedkeys(":EchoCond\<CR>", 'xt')
+  call assert_equal('true', g:result)
+
   delcommand EchoCond
   unlet g:result