Bug: ob-plantuml: using @start prefix in buffer returns nil body due to nil assignments [N/A (N/A @ /nix/store/1hmx5h1kn25p7s31h31dz7g3kn1dc6qx-emacs-packages-deps/share/emacs/site-lisp/org/)]

[Terje Larsen <terlar@gmail.com>]

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I noticed that I cannot run C-c C-c to convert plantuml source blocks
since the commit (4e854974be9788f029f2d73f829c4d51d2b83faf), see changes
at:
https://code.orgmode.org/bzg/org-mode/commit/4e854974be9788f029f2d73f829c4d51d2b83faf#diff-9c9a333463276ff17be763269a5042f92a40817R73

If I have a simple source block as such as:
#+BEGIN_SRC plantuml :file output.png
@startuml
actor Foo1
actor Foo2
Foo1 -> Foo2 : Test
@enduml
#+END_SRC

When I try to run C-c C-c I receive the following error message:
org-babel-execute-src-block: Wrong type argument: char-or-string-p, nil

During my debugging I found out that assignments inside the function
org-babel-plantuml-make-body returns nil and that causes the function to
return nil which is not expected behavior. I remember this working
before, so I went looking through the history and noticed the addition
of this code. To me it looks like the logic is wrong. As the expression:
(org-babel-variable-assignments:plantuml params)

Could not possibly not return the body and without the body the content
will be invalid. I have verified that reverting this commit fixes the
issue and lets me generate PlantUML files.

Emacs  : GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.12)
Package: Org mode version N/A (N/A @ /nix/store/1hmx5h1kn25p7s31h31dz7g3kn1dc6qx-emacs-packages-deps/share/emacs/site-lisp/org/)