Skip redirect if no entry to redirect to

[ericphanson]

Fix #1693

untested..

[ericphanson]

Added a test and checked that it failed on master:

julia> include("test/htmlwriter.jl")
WARNING: replacing module HTMLWriterTests.
HTMLWriter: Error During Test at /Users/eph/.julia/dev/Documenter/test/htmlwriter.jl:26
  Got exception outside of a @test
  BoundsError: attempt to access 0-element Vector{String} at index [1]
  Stacktrace:
    [1] getindex
      @ ./array.jl:835 [inlined]
    [2] first
      @ ./abstractarray.jl:398 [inlined]
    [3] #261
      @ ~/.julia/dev/Documenter/src/Writers/HTMLWriter.jl:1423 [inlined]
    [4] open(::Documenter.Writers.HTMLWriter.var"#261#262"{Vector{String}, String}, ::String, ::Vararg{String}; kwargs::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
      @ Base ./io.jl:330
    [5] open
      @ ./io.jl:328 [inlined]
    [6] generate_redirect_file(redirectfile::String, entries::Vector{String})
      @ Documenter.Writers.HTMLWriter ~/.julia/dev/Documenter/src/Writers/HTMLWriter.jl:1421
    [7] (::Main.HTMLWriterTests.var"#3#10")(tmpdir::String)
      @ Main.HTMLWriterTests ~/.julia/dev/Documenter/test/htmlwriter.jl:247
    [8] mktempdir(fn::Main.HTMLWriterTests.var"#3#10", parent::String; prefix::String)
      @ Base.Filesystem ./file.jl:750
    [9] mktempdir(fn::Function, parent::String) (repeats 2 times)
      @ Base.Filesystem ./file.jl:748
   [10] macro expansion
      @ ~/.julia/dev/Documenter/test/htmlwriter.jl:152 [inlined]
   [11] macro expansion
      @ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/Test/src/Test.jl:1282 [inlined]
   [12] top-level scope
      @ ~/.julia/dev/Documenter/test/htmlwriter.jl:27
   [13] include(fname::String)
      @ Base.MainInclude ./client.jl:451
   [14] top-level scope
      @ REPL[3]:1
   [15] eval
      @ ./boot.jl:373 [inlined]
   [16] eval_user_input(ast::Any, backend::REPL.REPLBackend)
      @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/REPL/src/REPL.jl:150
   [17] repl_backend_loop(backend::REPL.REPLBackend)
      @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/REPL/src/REPL.jl:241
   [18] start_repl_backend(backend::REPL.REPLBackend, consumer::Any)
      @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/REPL/src/REPL.jl:226
   [19] run_repl(repl::REPL.AbstractREPL, consumer::Any; backend_on_current_task::Bool)
      @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/REPL/src/REPL.jl:359
   [20] run_repl(repl::REPL.AbstractREPL, consumer::Any)
      @ REPL /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/REPL/src/REPL.jl:346
   [21] (::Base.var"#929#931"{Bool, Bool, Bool})(REPL::Module)
      @ Base ./client.jl:394
   [22] #invokelatest#2
      @ ./essentials.jl:716 [inlined]
   [23] invokelatest
      @ ./essentials.jl:714 [inlined]
   [24] run_main_repl(interactive::Bool, quiet::Bool, banner::Bool, history_file::Bool, color_set::Bool)
      @ Base ./client.jl:379
   [25] exec_options(opts::Base.JLOptions)
      @ Base ./client.jl:309
   [26] _start()
      @ Base ./client.jl:495
Test Summary: | Pass  Error  Total
HTMLWriter    |  130      1    131
ERROR: LoadError: Some tests did not pass: 130 passed, 0 failed, 1 errored, 0 broken.
in expression starting at /Users/eph/.julia/dev/Documenter/test/htmlwriter.jl:1