Fix named ranges behaviour if cidr_tpl_file variable not provided.

[miromichalicka]

Issue: If factories_config.cidr_tpl_file variable wasn't provided, the code failed.

│ Error: Invalid function argument
│ 
│   on .terraform/modules/internal_firewall/modules/net-vpc-firewall/main.tf line 50, in locals:
│   50:     can(var.factories_config.cidr_tpl_file) ? yamldecode(file(var.factories_config.cidr_tpl_file)) : {},
│     ├────────────────
│     │ while calling file(path)
│     │ var.factories_config.cidr_tpl_file is null
│ 
│ Invalid value for "path" parameter: argument must not be null.

Solution: Validate if the variable isn't null (variable's default value) and only then proceed to load the file.

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass