patch 9.0.1995: Invalid memory access with empty 'foldexpr'

Problem:  Invalid memory access when 'foldexpr' returns empty string.
Solution: Check for NUL.

closes: #13293

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index 398a0c2..cb29d43 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
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