[clang] [Clang] Give empty template parameter mapping an empty MLTAL (PR #164051)

[Corentin Jabot via cfe-commits]

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@@ -2050,6 +2059,11 @@ bool SubstituteParameterMappings::substitute(
   if (!N.hasParameterMapping())
     buildParameterMapping(N);
 
+  // Don't bother into substituting if we have determined the mapping maps to
+  // nothing.
----------------
cor3ntin wrote:

```suggestion
  // If the parameter mapping is empty, there is nothing to substitute.
```

https://github.com/llvm/llvm-project/pull/164051