SymbolFile: invert condition, remove unnecessary else (NFC)

Remove an unnecessary `else` condition and swap the condition to be
positive rather than negative to make simpler to follow.  NFC

Author: compnerd

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp

@@ -1636,10 +1636,9 @@ bool SymbolFileDWARF::GetFunction(const DWARFDIE &die, SymbolContext &sc) {
 lldb::ModuleSP SymbolFileDWARF::GetExternalModule(ConstString name) {
   UpdateExternalModuleListIfNeeded();
   const auto &pos = m_external_type_modules.find(name);
-  if (pos != m_external_type_modules.end())
-    return pos->second;
-  else
+  if (pos == m_external_type_modules.end())
     return lldb::ModuleSP();
+  return pos->second;
 }
 
 DWARFDIE