fix: Don't error if $OPTS is not yet defined in .zinit-compinit call

This codepath was triggered when deleting the whole `~/.zinit/plugins` dir: during recreation in `.zinit-prepare-home`, `.zinit-compinit &> /dev/null` would be called and the sourcing of `zinit.zsh` would error out with

```
+.zinit-compinit:2> [[.zinit-compinit:2: bad math expression: operand expected at `/Users/jan...'
 -n '' ]]
```

As the error was redirected to `/dev/null`, this never showed up. Any zinit install call during that session would not run any hooks as they were not yet registered. The next session would then be fine because the plugins dir would exist.

zinit-install.zsh

@@ -601,7 +601,8 @@ builtin source "${ZINIT[BIN_DIR]}/zinit-side.zsh" || {
 #
 # No arguments.
 .zinit-compinit() {
-    [[ -n ${OPTS[opt_-p,--parallel]} && $1 != 1 ]] && return
+    # This might be called during sourcing when setting up the plugins dir, so check that OPTS is actually existing
+    [[ -n $OPTS && -n ${OPTS[opt_-p,--parallel]} && $1 != 1 ]] && return
 
     emulate -LR zsh
     builtin setopt nullglob extendedglob warncreateglobal typesetsilent