bug#34147: 27.0.50; M-x describe-package groovy-mode breaks

[Xu Chunyang]

I've setup Melpa, C-h P groovy-mode breaks, here is the backtrace

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  #f(compiled-function (pkg) #<bytecode 0x45e6b941>)(groovy-mode)
  apply(#f(compiled-function (pkg) #<bytecode 0x45e6b941>) groovy-mode)
  describe-package-1(groovy-mode)
  describe-package(groovy-mode)
  funcall-interactively(describe-package groovy-mode)
  call-interactively(describe-package nil nil)
  command-execute(describe-package)

By evaluating describe-package-1 to get detailed Backtrace (I didn't
paste it since it is very long), it seems the error is caused by the
"last" expression of describe-package-1:

(insert readme-string)

when readme-string is nil.

I notice Melpa fails to provide the readme file, this URL is 404

https://melpa.org/packages/groovy-mode-readme.txt

--
In GNU Emacs 27.0.50
Repository revision: 436c225f1b8ca48fa7c7e769fe6b7055f37af95d
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1671
System Description:  Mac OS X 10.14.2