fix(rustic-babel): disable toolchain when invalid/unneeded

When the user installs Rust tools using a method other than rustup, e.g. using
an operating system's package manager, cargo generally has no support for
toolchain specification. In such case, the user can then `nil', or `""', so that
the rescpective functions in `rustic-babel' will remove the toolchain from
params, that is only toolchain has a valid value it uses toolchain.

See also: brotzeit#279 (comment)

rustic-babel.el

@@ -73,9 +73,18 @@ should be wrapped in which case we will disable rustfmt."
           (toolchain (cond ((eq toolchain-kw-or-string 'nightly) "+nightly")
                            ((eq toolchain-kw-or-string 'beta) "+beta")
                            ((eq toolchain-kw-or-string 'stable) "+stable")
+                           ((or (null rustic-babel-default-toolchain)
+                                (null (string-blank-p rustic-babel-default-toolchain)))
+                            nil)
                            (toolchain-kw-or-string (format "+%s" toolchain-kw-or-string))
                            (t (format "+%s" rustic-babel-default-toolchain))))
-          (params (list "cargo" toolchain "build" "--quiet"))
+          (params (remove nil (list "cargo"
+                                    (unless (or (null toolchain)
+                                                (= 0 (string-blank-p toolchain))
+                                                (equal "+" toolchain))
+                                      toolchain)
+                                    "build"
+                                    "--quiet")))
           (inhibit-read-only t))
      (rustic-compilation-setup-buffer err-buff dir 'rustic-compilation-mode)
      (when rustic-babel-display-compilation-buffer
@@ -126,7 +135,13 @@ execution with rustfmt."
 
            ;; run project
            (let* ((err-buff (get-buffer-create rustic-babel-compilation-buffer-name))
-                  (params (list "cargo" toolchain "run" "--quiet"))
+                  (params (remove nil (list "cargo"
+                                            (unless (or (null toolchain)
+                                                        (= 0 (string-blank-p toolchain))
+                                                        (equal "+" toolchain))
+                                              toolchain)
+                                            "run"
+                                            "--quiet")))
                   (inhibit-read-only t))
              (rustic-make-process
               :name rustic-babel-process-name