qmake project with empty SOURCES is producing broken MSVC makefile

1b46872cde7fb50652f55bfe1e286e6af5124200 which fixed another bug, introduced a different regression:

If any qmake project has SOURCES empty (could be different, but this is the symptom I saw), for example it is calling a doxygen command but not really building a library, the created makefile is broken. This was perfectly working before the above commit.

Minimal example:

TEMPLATE = lib
CONFIG += staticlib
CONFIG -= qt

isEmpty(DOXYGEN_BIN) {
        message("Doxygen not found")
} else {
    # Doxygen command simplified for QTBUG report
    doxy.commands = echo test
    doxy.CONFIG = no_link target_predeps
    doxy.depends = $$PWD/$$DOXYFILE
    doxy.input = DOXY_INPUT
    doxy.name = DOXY
    doxy.output = html/index.html

    QMAKE_EXTRA_COMPILERS += doxy
}

produces a broken makefile (the list of objects does not exist and make tries to interpret the lines below as objects)

$(DESTDIR_TARGET):  $(OBJECTS) 
	$(LIBAPP) $(LIBFLAGS) /OUT:$(DESTDIR_TARGET) @<<
	  <<

which does not properly finish the block.

With 5.8 or reverting the above commit produces

$(DESTDIR_TARGET):  $(OBJECTS) 
	$(LIBAPP) $(LIBFLAGS) /OUT:$(DESTDIR_TARGET) @<<
	  $(OBJECTS)
<<

Workaround is to add a dummy cpp file to SOURCES which is ok sometimes and does not work in all cases. People with a similar use case might run into strange build errors and it is not easy to spot why.