home: vim: only use 'null-ls' sources if available

This avoids the big red warning on each file that tries to use those
sources...

home/vim/after/ftplugin/python.vim

@@ -4,14 +4,30 @@ call ftplugined#check_undo_ft()
 " Set-up LSP, linters, formatters
 lua << EOF
 local null_ls = require("null-ls")
+local utils = require("ambroisie.utils")
+
 null_ls.register({
-    null_ls.builtins.diagnostics.flake8,
-    null_ls.builtins.diagnostics.mypy,
-    null_ls.builtins.diagnostics.pylint,
+    null_ls.builtins.diagnostics.flake8.with({
+        -- Only used if available
+        condition = utils.is_executable_condition("flake8"),
+    }),
+    null_ls.builtins.diagnostics.mypy.with({
+        -- Only used if available
+        condition = utils.is_executable_condition("mypy"),
+    }),
+    null_ls.builtins.diagnostics.pylint.with({
+        -- Only used if available
+        condition = utils.is_executable_condition("pylint"),
+    })
     null_ls.builtins.formatting.black.with({
         extra_args = { "--fast" },
+        -- Only used if available
+        condition = utils.is_executable_condition("black"),
+    }),
+    null_ls.builtins.formatting.isort.with({
+        -- Only used if available
+        condition = utils.is_executable_condition("isort"),
     }),
-    null_ls.builtins.formatting.isort,
 })
 EOF