Add null safety checks in GraphQL response handling

[MariusStorhaug]

Request

Problem: Several private functions that use GraphQL to fetch a single resource do not check for $null before constructing class instances. When the queried resource does not exist (e.g., a deleted repository, a non-existent enterprise, or a release with no matching tag), the GraphQL response returns null for that field, causing constructor errors.

Context

With the introduction of partial error handling in Invoke-GitHubGraphQLQuery (see #557), GraphQL queries can now return partial data where some fields are null alongside errors. Functions must guard against null values to avoid runtime exceptions when constructing typed objects.

Acceptance criteria

• All affected functions check for null before calling class constructors
• Functions silently return nothing (no output) when the queried resource is null
• No constructor errors when querying non-existent repositories, enterprises, or releases

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Technical decisions

Pattern: Assign the GraphQL result to a variable, then wrap the constructor call in an if ($variable) check. This is consistent with PowerShell idioms and keeps the code readable.

Affected functions:

Function	File	Null field
Get-GitHubRepositoryByName	src/functions/private/Repositories/Get-GitHubRepositoryByName.ps1	$_.repositoryOwner.repository
Get-GitHubMyRepositoryByName	src/functions/private/Repositories/Get-GitHubMyRepositoryByName.ps1	$_.viewer.repository
Get-GitHubEnterpriseByName	src/functions/private/Enterprise/Get-GitHubEnterpriseByName.ps1	$enterpriseResult.enterprise
Get-GitHubReleaseAssetByTag	src/functions/private/Releases/Assets/Get-GitHubReleaseAssetByTag.ps1	$_.repository.release
Get-GitHubReleaseAssetFromLatest	src/functions/private/Releases/Assets/Get-GitHubReleaseAssetFromLatest.ps1	$_.repository.latestRelease

No new tests: These are defensive null checks on private functions. They will be exercised by existing integration tests when querying non-existent resources.

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Implementation plan

Core changes

• Get-GitHubRepositoryByName — guard $_.repositoryOwner.repository before constructing [GitHubRepository]
• Get-GitHubMyRepositoryByName — guard $_.viewer.repository before constructing [GitHubRepository]
• Get-GitHubEnterpriseByName — guard $enterpriseResult.enterprise before constructing [GitHubEnterprise]
• Get-GitHubReleaseAssetByTag — guard $_.repository.release before processing assets and pagination
• Get-GitHubReleaseAssetFromLatest — guard $_.repository.latestRelease before processing assets and pagination