Don't allow empty namespaces.

An empty namespace (""), or a string made up of only whitespace,
isn't very descriptive and may cause issues with debugging in the
future. This blocks clients from using such namespaces in the first
place.

Bug: 276313158
Test: atest CtsJobSchedulerTestCases:JobSchedulingTest
Change-Id: I2b8da58fc1836b8547270b2c6f39fe44fd34f936