Add nullptr check for captureListener in captureScreenCommon

RegionSamplingThread::captureSample intentionally passes null, which is
already checked for below, but missing in this failure case of
`!renderArea`

Bug: 259021062
Test: presubmits (cost/benefit ratio of adding a new test for this seems
high, given current testing flows and simplicity of change)

Change-Id: I460539404be7a7ae434812aa1f583bba6247a812