LibCpp: Don't include parameter type in FunctionType::to_string if null

The type of a function parameter can be null if we failed to parse it.
In such a case, calling to_string() on a FunctionType node used to cause
a null dereference.

This caused the language server to crash when processing
AK/StdLibExtraDetails.h

Userland/Libraries/LibCpp/AST.cpp

@@ -124,7 +124,8 @@ String FunctionType::to_string() const
  first = false;
  else
  builder.append(", ");
- builder.append(parameter.type()->to_string());
+ if (parameter.type())
+ builder.append(parameter.type()->to_string());
  if (parameter.name() && !parameter.full_name().is_empty()) {
  builder.append(" ");
  builder.append(parameter.full_name());