Fix IgxGridSelectionService.generateRange crash on drag-select released outside grid

[mvanhorn]

Closes #17335

Description

generateRange no longer throws when the selection anchor is null; it returns an empty range for that case instead of dereferencing the missing anchor.

Motivation / Context

When the range generator was called before a selection anchor was set (a null anchor), it dereferenced the anchor and threw, surfacing as a runtime crash in the consuming grid/selection logic. Guarding the null anchor returns a sensible empty range and avoids the crash. Closes #17335.

Type of Change (check all that apply):

• Bug fix

Component(s) / Area(s) Affected:

Selection / range generation utility.

How Has This Been Tested?

• Unit tests

Test Configuration:

• Angular version: repository default
• Browser(s): N/A (unit tests)
• OS: macOS

Screenshots / Recordings

N/A — non-visual bug fix.

Checklist:

• Added focused regression tests for the null-anchor case.

AI was used for assistance.