fix: roslyn doesn't attach to cs buffer when .sln is not present

The condition `not sln or not csproj` in sln.utils.root_dir
was causing roslyn not to attach to cs buffer if one of
.csproj or .sln was not found via `vim.fs.root_dir`

This behavior was introduced relatively recently in
d121ca7

The fix updates the condition to allow for only one of these being
found (the rest of the code already seems to account for it). After
updating the condition, find_files_with_extension(sln, ".sln") was
breaking as well, so I wrapped those calls in their own separate
conditions based on if .csproj or .sln root directories were found.

Existing github issue: seblyng#87

address pr feedback: early return on no sln

lua/roslyn/sln/utils.lua

@@ -41,17 +41,23 @@ function M.root_dir(buffer, broad_search)
         return name:match("%.csproj$") ~= nil
     end)
 
-    if not sln or not csproj then
+    if not sln and not csproj then
         return {}
     end
 
     local projects = csproj and { files = find_files_with_extension(csproj, ".csproj"), directory = csproj } or nil
 
+    if not sln then
+        return {
+            solutions = nil,
+            projects = projects,
+        }
+    end
+
     if broad_search then
         local solutions = vim.fs.find(function(name, _)
             return name:match("%.sln$")
         end, { type = "file", limit = math.huge, path = sln })
-
         return {
             solutions = solutions,
             projects = projects,