style(compiler): reformat long condition for stylua

Problem: for long-running processes (typst watch), M.compile() calls
the opener immediately after vim.system() returns, before the process
has produced any output. On first run the output file does not exist
yet, so the opener is called on a nonexistent path.

Solution: guard the open block with vim.uv.fs_stat so it only fires
if the output file already exists at spawn time.

lua/preview/compiler.lua

@@ -147,7 +147,12 @@ function M.compile(bufnr, name, provider, ctx)
       end)
     )
 
-    if provider.open and not opened[bufnr] and output_file ~= '' then
+    if
+      provider.open
+      and not opened[bufnr]
+      and output_file ~= ''
+      and vim.uv.fs_stat(output_file)
+    then
       if provider.open == true then
         vim.ui.open(output_file)
       elseif type(provider.open) == 'table' then