fix: only replace path with env var if value defined, otherwise leave $ character in the path

Author: bryankenote

lua/diffview/path.lua

@@ -295,8 +295,8 @@ function PathLib:expand(path)
 
   for i = idx, #segments do
     local env_var = segments[i]:match("^%$(%S+)$")
-    if env_var then
-      segments[i] = uv.os_getenv(env_var) or env_var
+    if env_var and uv.os_getenv(env_var) ~= nil then
+      segments[i] = uv.os_getenv(env_var)
     end
   end