${b+s ''}

[sunnycemetery]

I would like to include a null string as part of a parameter expansion word:

> mapfile -t${bNullDelimited+d ''}

When bNullDelimited is set, I expect this to expand to:

> mapfile -td ''

However, it expands to:

> mapfile -td

Note that a lone null string is successfully expanded:

> mapfile -t${bNullDelimited+d} ${bNullDelimited+''}

The bash gurus in my neck of the woods consider this behavior unexpected.\ 
Is it a bug?

5.0.0(1)-release