Ignore missing JAR in native installations (allows installation of jdtls through NixOS)

Do not check for the existence of a JAR if a native command is preferred and there is a suitable executable. I was hit by this issue on NixOS which puts the JAR at a different location than the auto detection of this package expects it. The alternative, setting `lsp-java-server-install-dir` such that the JAR instead of the executable is found, leads to a failure. When the JAR is launched directly, jdtls tries to write to the Nix Store which is read-only. Launching the jdtls executable provided by nixpkgs works perfectly though. The error I encountered was exactly the same as described in #421. Hence, I believe this PR fixes that issue without overriding any private functions in the user configuration (see below). As I understand it, this is also the alternative solution for #487 as suggested by @LuigiPiucco in https://github.com/emacs-lsp/lsp-java/issues/487#issuecomment-2383307915 In case this is desired, I could also add a default fallback in `lsp-java--locate-server-command` if `lsp-java-jdt-ls-prefer-native-command` is `t` and `lsp-java-jdt-ls-command` is found in `exec-path`. ## Working NixOS configuration When installing `jdtls` globally, my working configuration after this PR looks like this: ```elisp (setq lsp-java-jdt-ls-prefer-native-command t) (setq lsp-java-server-install-dir (f-dirname (f-dirname (executable-find lsp-java-jdt-ls-command)))) ``` Note that `jdt-language-server` was renamed to `jdtls` in nixpkgs. In case you are using some old nixpkgs version, you also need ```elisp (setq lsp-java-jdt-ls-command "jdt-language-server") ``` before the first two lines. If you want to use nix shells with direnv, you can follow the [guide by @dschrempf](https://dschrempf.github.io/emacs/2023-03-02-emacs-java-and-nix/) and use the following Emacs configuration: ```elisp (defun my-set-lsp-path () (let ((jdtls-exe (executable-find lsp-java-jdt-ls-command))) (when jdtls-exe (setq lsp-java-jdt-ls-prefer-native-command t) (setq lsp-java-server-install-dir (f-dirname (f-dirname jdtls-exe)))))) (after! cc-mode (add-hook 'java-mode-hook #'my-set-lsp-path)) ```