Setting a record to an empty string is the same as making it null.

dlls/msi/record.c

@@ -455,7 +455,7 @@ UINT MSI_RecordSetStringA( MSIRECORD *rec, unsigned int iField, LPCSTR szValue )
         return ERROR_INVALID_FIELD;
 
     MSI_FreeField( &rec->fields[iField] );
-    if( szValue )
+    if( szValue && szValue[0] )
     {
         len = MultiByteToWideChar( CP_ACP, 0, szValue, -1, NULL, 0 );
         str = HeapAlloc( GetProcessHeap(), 0, len*sizeof(WCHAR) );
@@ -501,7 +501,7 @@ UINT MSI_RecordSetStringW( MSIRECORD *rec, unsigned int iField, LPCWSTR szValue
 
     MSI_FreeField( &rec->fields[iField] );
 
-    if( szValue )
+    if( szValue && szValue[0] )
     {
         len = lstrlenW(szValue) + 1;
         str = HeapAlloc( GetProcessHeap(), 0, len*sizeof (WCHAR));

dlls/msi/tests/record.c

@@ -123,6 +123,10 @@ void test_msirecord(void)
     ok(r == ERROR_SUCCESS, "Failed to set null string at 0\n");
     r = MsiRecordIsNull(h, 0);
     ok(r == TRUE, "null string not null field\n");
+    r = MsiRecordSetString(h, 0, "");
+    ok(r == ERROR_SUCCESS, "Failed to set empty string at 0\n");
+    r = MsiRecordIsNull(h, 0);
+    ok(r == TRUE, "null string not null field\n");
     r = MsiRecordSetString(h,0,str);
     ok(r == ERROR_SUCCESS, "Failed to set string at 0\n");
     r = MsiRecordGetInteger(h, 0);