qdoc: Distinguish between undefined and defined-as-empty config vars

Make Config::getString() return an empty string for variables that are
defined as empty, while continuing to return a null string for
undefined ones.

Use a defined-as-empty 'moduleheader' variable:

  moduleheader =

to mark a documentation project that doesn't need a precompiled header
to be built.

Change-Id: I2ccb631c73a1b91dc35955e5f21648db88915064
Reviewed-by: Martin Smith <martin.smith@qt.io>