cmGlobalGenerator: Fix CheckTargetsForMissingSources after refactoring

Refactoring in commit 01b2d6ab (Modernize: Use ranged for-loops when
possible, 2019-02-07, v3.15.0-rc1~575^2) accidentally changed a loop
condition in this method from "keep iterating if srcs.empty()" to
"stop iterating if srcs.empty()".  Switch it back.

The bug could only manifest in very subtle conditions in a multi-config
generator.  Add one such case to the test suite.

Fixes: #20706