Handle case where `pyenv-commands --sh` returns nothing (#2908)

In exceptional cases (custom installation, malfunctions elsewhere), `pyenv-commands --sh` may return nothing.
In non-Fish, this would cause "syntax error near unexpected token `)'" in `pyenv()`.

Bash does not allow to specify a `case` option that would never match.
This works around it by defaulting to `/`. Commands, being filenames, can never match it.
In Fish, nothing needs to be done: it apparently does interpret a `case` without argument as one that never matches.

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