fix: handle null undrainable_node_behavior in precondition validation

[lonegunmanb]

Fix

Resolves #760

Problem

When undrainable_node_behavior is explicitly set to null inside upgrade_settings for extra node pools, terraform plan crashes with:

Error: Invalid function argument contains(list, value) Invalid value for "value" parameter: argument must not be null.

Root Cause

In extra_node_pool.tf lines 183 and 350, the precondition uses:

condition = ... || contains(["Cordon", "Schedule"], try(each.value.upgrade_settings.undrainable_node_behavior, ""))

Two Terraform behaviors combine to cause this:

1. try() passes null through — accessing a null-valued attribute is not an error, so try(null, "") returns null, not "".
2. Terraform does not short-circuit || — all operands are evaluated before computing the logical result, so contains() receives null even though an earlier clause would make the expression true.

Fix

Wrap with coalesce() to convert null to "" before passing to contains():

contains(["Cordon", "Schedule"], coalesce(try(each.value.upgrade_settings.undrainable_node_behavior, ""), ""))

Also hardened a similar theoretical edge case in main.tf (blob driver subnet service endpoint check).

Changes

• extra_node_pool.tf: Fix null-safety in precondition on lines 183 and 350 (both node_pool_create_before_destroy and node_pool_create_after_destroy)
• main.tf: Harden service_endpoints null-safety in blob driver check (line 748)