Clang: Use system PCH if projectpart PCH does not exists

Then entry can be empty or null, so we test for both.

Task-number: QTCREATORBUG-21978
Change-Id: Ic824d897b3c38051a2304c9487ea5f2f7c9eeef8
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangpchmanagerbackend/source/precompiledheaderstorage.h

@@ -160,8 +160,9 @@ public:
         "SELECT systemPchPath FROM precompiledHeaders WHERE projectPartId = (SELECT projectPartId "
         "FROM projectParts WHERE projectPartName = ?)",
         m_database};
-    mutable ReadStatement m_getPrecompiledHeader{"SELECT projectPchPath, projectPchBuildTime FROM "
-                                                 "precompiledHeaders WHERE projectPartId = ?",
+    mutable ReadStatement m_getPrecompiledHeader{
+        "SELECT ifnull(nullif(projectPchPath, ''), systemPchPath), "
+        "projectPchBuildTime FROM precompiledHeaders WHERE projectPartId = ?",
         m_database};
 };