More jldoctest issues with format_docstrings

[mweerden]

When format_docstrings is set to true, I see the following issues:

Empty jldoctest:

"""
```jldoctest
```
"""
f() = 1

Gives:

┌ Warning: Error occurred during prettification
│   line = 4
│   offset = 4
└ @ JuliaFormatter ~/.julia/packages/JuliaFormatter/FPeem/src/JuliaFormatter.jl:252
ERROR: BoundsError: attempt to access 0-element Vector{CSTParser.EXPR} at index [1]

Triple quotes inside jldoctest:

"""
```jldoctest
\"""
```
"""
f() = 1

Gives:

┌ Warning: Error occurred during prettification
│   line = 5
│   offset = 4
└ @ JuliaFormatter ~/.julia/packages/JuliaFormatter/FPeem/src/JuliaFormatter.jl:252
ERROR: Parsing error for input occurred on line 1, offset: 1

Filter with backslashes:

"""
```jldoctest; filter = r"\\x1b\\[[^m]*m"
julia> 1;
```
"""
f() = 1

Is broken after formatting because backslashes are "evaluated":

"""
```jldoctest; filter = r"\x1b\[[^m]*m"
julia> 1;

```
"""
f() = 1