Handle case where pyenv-commands --sh returns nothing

When running `eval "$(pyenv init -)"` to initialize `pyenv`, the command
generates a shell script to be executed. When not using the `fish`
shell, this script contains a `case` statement where the first case
matches against the output of `pyenv-commands --sh` and the second case
handles everything else. However, if no commands are returned, then the
matching expression will be only `)`, which is invalid Bash.

I have solved this (on the advice of @ native-api) by using the string
`/` if there are no commands to include. `/` is the only character in
Linux/POSIX besides `\0` that a filename cannot contain, so it is safe
to use as a pattern that will never match. To catch any regressions, I
have added a test to catch unexpected changes in the generated `case`
statement. I copied the `create_executable` function from `latest.bats`
because I needed to shim `pyenv-commands`.

I do not know if `pyenv-commands --sh` can ever return nothing under
normal conditions, but it happened to me due to completely unrelated
problems in underlying Unix utilities called by `pyenv`. Although this
bug will rarely occur, I feel that this increased robustness will be
useful.

libexec/pyenv-init

@@ -297,7 +297,7 @@ EOS
   fi
 
   case "\$command" in
-  ${commands[*]})
+  ${commands[*]:-/})
     eval "\$(pyenv "sh-\$command" "\$@")"
     ;;
   *)

test/init.bats

@@ -2,6 +2,18 @@
 
 load test_helper
 
+setup() {
+  export PATH="${PYENV_TEST_DIR}/bin:$PATH"
+}
+
+create_executable() {
+  local name="$1"
+  local bin="${PYENV_TEST_DIR}/bin"
+  mkdir -p "$bin"
+  sed -Ee '1s/^ +//' > "${bin}/$name"
+  chmod +x "${bin}/$name"
+}
+
 @test "creates shims and versions directories" {
   assert [ ! -d "${PYENV_ROOT}/shims" ]
   assert [ ! -d "${PYENV_ROOT}/versions" ]
@@ -167,6 +179,24 @@ echo "\$PATH"
   assert_line '  case "$command" in'
 }
 
+@test "outputs sh-compatible case syntax" {
+  create_executable pyenv-commands <<!
+#!$BASH
+echo -e 'activate\ndeactivate\nrehash\nshell'
+!
+  run pyenv-init - bash
+  assert_success
+  assert_line '  activate|deactivate|rehash|shell)'
+
+  create_executable pyenv-commands <<!
+#!$BASH
+echo
+!
+  run pyenv-init - bash
+  assert_success
+  assert_line '  /)'
+}
+
 @test "outputs fish-specific syntax (fish)" {
   run pyenv-init - fish
   assert_success