ProjectExplorer: Stop caching useless macro inspection report

If there is a manually added compiler in "Menu: Tools > Options > Kits >
Compilers" and either

 (1) "Apply" is clicked after Qt Creator start
 (2) or an invalid compiler path is inserted and "Apply" is clicked

we don't have any macros. Thus, avoid populating the macro cache for
this case.

This avoids triggering

  SOFT ASSERT: "false && "__cplusplus is not predefined, assuming latest
  C++ we support."" in file toolchain.cpp, line 324

Change-Id: I23d20a24385824f4fa2ca6f39a8a04dc0eb0c308
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/projectexplorer/gcctoolchain.cpp

@@ -1083,6 +1083,10 @@ void GccToolChainConfigWidget::applyImpl()
     tc->setDisplayName(displayName); // reset display name
     tc->setPlatformCodeGenFlags(splitString(m_platformCodeGenFlagsLineEdit->text()));
     tc->setPlatformLinkerFlags(splitString(m_platformLinkerFlagsLineEdit->text()));
+
+    if (m_macros.isEmpty())
+        return;
+
     tc->m_predefinedMacrosCache
         ->insert(tc->platformCodeGenFlags(),
                  ToolChain::MacroInspectionReport{m_macros,