Update NPMSTest to avoid crashes during tear down.

If the policyDir is not properly initialized, the test can crash during
tear down. This can lead to subsequent tests failing as the other tear
down steps will be skipped. To fix this we just check if it is null
before attempting to delete files.

Bug: 432481295
Test: atest services/tests/servicestests/src/com/android/server/net/NetworkPolicyManagerServiceTest.java
Flag: TEST_ONLY
Change-Id: I68ab305b938df0d54949b7d3951c9b7d2a3f3562