Check that JULIA_PKG_SERVER is non-empty

[mortenpi]

Currently, if JULIA_PKG_SERVER is set but empty, you get this:

JULIA_PKG_SERVER= julia -e 'using PkgAuthentication; PkgAuthentication.authenticate()'
ERROR: MethodError: no method matching rstrip(::Nothing, ::Char)
Closest candidates are:
  rstrip(::Any, ::AbstractString) at strings/util.jl:382
  rstrip(::AbstractString, ::Union{AbstractChar, Tuple{Vararg{AbstractChar}}, Set{<:AbstractChar}, AbstractVector{<:AbstractChar}}) at strings/util.jl:389
Stacktrace:
 [1] authenticate(; auth_suffix::Nothing, force::Nothing, tries::Nothing)
   @ PkgAuthentication ~/.julia/packages/PkgAuthentication/YFLW0/src/PkgAuthentication.jl:103
 [2] authenticate()
   @ PkgAuthentication ~/.julia/packages/PkgAuthentication/YFLW0/src/PkgAuthentication.jl:77
 [3] top-level scope
   @ none:1

This is because Pkg.pkg_server() === nothing if JULIA_PKG_SERVER is set, but empty.

With this you get

$ JULIA_PKG_SERVER= julia --project -e 'using PkgAuthentication; PkgAuthentication.authenticate()'
ERROR: The `JULIA_PKG_SERVER` environment variable must be set and non-empty
Stacktrace:
 [1] _assert_pkg_server_env_var_is_set()
   @ PkgAuthentication ~/jc/PkgAuthentication.jl/src/PkgAuthentication.jl:28
 [2] authenticate(; auth_suffix::Nothing, force::Nothing, tries::Nothing)
   @ PkgAuthentication ~/jc/PkgAuthentication.jl/src/PkgAuthentication.jl:100
 [3] authenticate()
   @ PkgAuthentication ~/jc/PkgAuthentication.jl/src/PkgAuthentication.jl:77
 [4] top-level scope
   @ none:1

which is a bit more informative.

[codecov-commenter]

Codecov Report

Merging #23 (141053e) into master (87517b4) will decrease coverage by 2.97%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   73.58%   70.61%   -2.97%     
==========================================
  Files           1        1              
  Lines         212      211       -1     
==========================================
- Hits          156      149       -7     
- Misses         56       62       +6     

Impacted Files	Coverage Δ
src/PkgAuthentication.jl	70.61% <50.00%> (-2.97%)	⬇️

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