[REG 5.13->5.14] qdoc's single-exec wll not generate documentation in special case

For some particular configuration qdoc will not generate documentatoin.

top reproduce use following data attached and run

rm -rf ~/testaaa; mkdir -p ~/testaaa; qdoc data/conf/single_exec.qdocconf --single-exec --outputdir ~/testaaa

Note that qdoc successfully creates documentation in older releases ( 5.12 for example )