Add support for empty strings

[Wynand]

I ran into an issue where empty strings would cause an error:

julia> ss""
ERROR: ArgumentError: 0 is <= 0
Stacktrace:
 [1] get_type(maxlen::Int64; types::NTuple{7, DataType})
   @ ShortStrings ~/git-repos/julia_packages/ShortStrings.jl/src/base.jl:309
 [2] ShortString(str::String, maxlen::Int64; types::NTuple{7, DataType})
   @ ShortStrings ~/git-repos/julia_packages/ShortStrings.jl/src/base.jl:322
 [3] ShortString(str::String, maxlen::Int64)
   @ ShortStrings ~/git-repos/julia_packages/ShortStrings.jl/src/base.jl:322
 [4] top-level scope
   @ REPL[2]:1

After some brief digging I couldn't find any documented reason why empty strings shouldn't be allowed and it seemed simple enough to add support

[oxinabox]

please also bump the version, so i can tag a release

[Wynand]

please also bump the version, so i can tag a release

I bumped this as a bugfix, is that correct?

[oxinabox]

This is pre-1.0 so there is only breaking and nonbreaking, and this is indeed nonbreaking