[ruby-core:108697] [Ruby master Bug#18805] IO::Buffer is inconsistent when returning a string from an empty buffer

Issue #18805 has been reported by procmarco (Marco Concetto Rudilosso).

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Bug #18805: IO::Buffer is inconsistent when returning a string from an empty buffer
https://bugs.ruby-lang.org/issues/18805

* Author: procmarco (Marco Concetto Rudilosso)
* Status: Open
* Priority: Normal
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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I’ve been using `IO::Buffer` and I found it to be slightly inconsistent when it comes to returning empty string for empty buffers.
for example, a slice of an allocated buffer with `size = 0`, returns `""` with `get_string`, as an example:

```ruby
buffer = IO::Buffer.new(5)
empty_buffer = buffer.slice(0,0)
puts empty_buffer.size # this prints 0
empty_buffer.get_string # this returns ""
```

but, if you create a buffer with IO::Buffer.new(0) then it stops working

```ruby
empty_buffer = IO::Buffer.new(0)
puts empty_buffer.size # this prints 0
empty_buffer.get_string # this raises The buffer is not allocated! (IO::Buffer::AllocationError)
```

Is this working as intended? It would be good I think to have a consistent experience where the base case (buffer with size 0) always returns an empty string.
I have a prototype of a possible patch I could send upstream to fix it, which would check the size of the buffer and if 0 it would always return an empty string.



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