patch 8.2.4869: expression in command block does not look after NL

Problem:    Expression in command block does not look after NL.
Solution:   Skip over NL to check what follows. (closes #10358)

src/testdir/test_usercommands.vim

@@ -694,6 +694,20 @@ func Test_usercmd_with_block()
   call v9.CheckScriptSuccess(lines)
   delcommand HelloThere
 
+  let lines =<< trim END
+      command EchoCond {
+          const test: string = true
+              ? 'true'
+              : 'false'
+          g:result = test
+      }
+      EchoCond
+  END
+  call v9.CheckScriptSuccess(lines)
+  call assert_equal('true', g:result)
+  delcommand EchoCond
+  unlet g:result
+
   let lines =<< trim END
       command BadCommand {
          echo  {