Description of problem:
NPE is thrown, when resource is added to KieFileSystem with unspecified path and rule in xls contains timer and calendar. Detailed description is below in Steps to Reproduce.

Version-Release number of selected component (if applicable):
Drools 6.0.0-SNAPSHOT
Drools 6.0.0.ER4

How reproducible:
Pull request #276 submited in Drools.

Steps to Reproduce:
1. Create test with one kieSession. At initialization, write xls resource into KieFileSystem with unspecified path. Create and set calendar. Set clock to some day, when the rule do not fire. Finally fireAllRules.
2. Create xls decision table with one rule. Rule should contain timer ("int" timer and "repeat interval" value should be zero) and calendar, which make rule active in one day of week.
3. Run test.

Actual results:
NPE is thrown.

Expected results:
Test should pass.

Additional info:
When resource is added with specified path into kfs, test pass.
When clock are set to day when rule should fire, test pass.
When "repeat interval" value is not zero, test pass.