msi: Treat empty string as NULL in MsiGetSummaryInformationW.

dlls/msi/suminfo.c

@@ -469,7 +469,7 @@ UINT WINAPI MsiGetSummaryInformationW( MSIHANDLE hDatabase,
     if( !pHandle )
         return ERROR_INVALID_PARAMETER;
 
-    if( szDatabase )
+    if( szDatabase && szDatabase[0] )
     {
         LPCWSTR persist = uiUpdateCount ? MSIDBOPEN_TRANSACT : MSIDBOPEN_READONLY;
 

dlls/msi/tests/suminfo.c

@@ -86,7 +86,18 @@ static void test_suminfo(void)
     ok(r == ERROR_INVALID_PARAMETER, "MsiGetSummaryInformation wrong error\n");
 
     r = MsiGetSummaryInformation(hdb, NULL, 0, &hsuminfo);
-    ok(r == ERROR_SUCCESS, "MsiGetSummaryInformation failed\n");
+    ok(r == ERROR_SUCCESS, "MsiGetSummaryInformation failed %u\n", r);
+
+    r = MsiCloseHandle(hsuminfo);
+    ok(r == ERROR_SUCCESS, "MsiCloseHandle failed\n");
+
+    r = MsiGetSummaryInformation(0, "", 0, &hsuminfo);
+    todo_wine
+    ok(r == ERROR_INSTALL_PACKAGE_INVALID || r == ERROR_INSTALL_PACKAGE_OPEN_FAILED,
+       "MsiGetSummaryInformation failed %u\n", r);
+
+    r = MsiGetSummaryInformation(hdb, "", 0, &hsuminfo);
+    ok(r == ERROR_SUCCESS, "MsiGetSummaryInformation failed %u\n", r);
 
     r = MsiSummaryInfoGetPropertyCount(0, NULL);
     ok(r == ERROR_INVALID_HANDLE, "getpropcount failed\n");