fix compilation of elisp forms with empty bodies

fix compilation of elisp forms with empty bodies

* module/language/elisp/compile-tree-il.scm (generate-let)
  (generate-let*, compile-lambda, compile-with-added-symbols)
  (compile-progn, compile-if): Return nil if the form's body is empty.
* test-suite/tests/elisp-compiler.test ("Sequencing")["empty progn"]:
  New test.
  ("Conditionals")["if with no else"]: New test.
  ("Let and Let*")["empty let, empty let*"]: New test.
  ("Lambda Expressions")["empty lambda"]: New test.