Autogen: Fix a bug in setting the UIC source group

If the relative path to a UI file was empty then the source group for the corresponding autogen header file would not apply because of a double slash in the path.
37 jobs for !5978 with autouic-source-group-fixes in 87 minutes and 4 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Build
passed build:centos6-x86_64 #5674139
cmake build docker linux manual

00:02:21

passed build:debian10-iwyu #5674136
cmake build docker linux

00:02:41

passed build:fedora33-ninja #5674140
cmake build docker linux linux-3.17 manual

00:01:19

passed build:fedora33-sphinx #5674138
cmake build docker linux linux-3.17

00:05:05

passed build:fedora33-tidy #5674137
linux-3.17 cmake build docker linux

00:03:55

passed build:macos-x86_64-makefiles #5674142
cmake macos shell xcode-12.4 nonconcurrent manual

00:07:15

passed build:macos-x86_64-ninja #5674141
cmake macos shell xcode-12.4 nonconcurrent manual

00:02:56

passed build:windows-vs2019-x64-ninja #5674143
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent manual

00:02:56

 
  Test
passed test:fedora33-ninja #5674144
cmake docker linux linux-3.17 x11

00:04:57

passed test:macos-x86_64-makefiles #5674146
cmake macos shell xcode-12.4 nonconcurrent

00:18:34

passed test:macos-x86_64-ninja #5674145
cmake macos shell xcode-12.4 nonconcurrent

00:09:38

passed test:windows-vs2019-x64-ninja #5679338
windows cmake shell vs2019 nonconcurrent msvc-19.28-16.9

00:19:30

failed test:windows-vs2019-x64-ninja #5674147
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent

00:16:12

 
  Test Ext
passed test:cuda10.2-nvidia #5674150
cmake cuda-rt docker linux

00:01:56

passed test:debian10-ninja #5674148
cmake build docker linux linux-3.17

00:08:52

passed test:fedora33-makefiles #5674149
cmake build docker linux linux-3.17

00:03:00

passed test:fedora33-ninja-multi #5674151
cmake build docker linux linux-3.17

00:03:13

passed test:macos-x86_64-xcode #5674152
cmake macos shell xcode-12.4 concurrent

00:58:07

passed test:windows-vs2019-x64 #5674153
cmake windows shell vs2019 msvc-19.28-16.9 concurrent

00:11:12

 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager