Fix panic while validating template

[RamanaReddy0M]

Proposed changes

• closes panic: interface conversion: interface {} is nil, not string [recovered] #4922

cent-nuclei-templates/test.yaml:

id: cve-2019-3799
info:
  name: Spring Cloud Config Server Path Traversal
  severity: High
  author: pdteam

variables:
  - path: test/pathtraversal/master

requests: 
  - method: GET
    path:
      - "{{BaseURL}}/{{path}}"

TEST:

✗ nuclei -t cent-nuclei-templates/test.yaml -validate

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[RamanaReddy0M]

Note: Exited without panics.