Filter out null labels in plugin sales SelectFilter options

[simonhamp]

Summary

• Added whereNotNull query constraints to all three SelectFilter relationship definitions in PluginSalesResource to prevent null labels from being passed to Filament's Select::isOptionDisabled() method
• Added test coverage for the plugin sales page, including a regression test with a null-name plugin

Problem

When a Plugin record has a null name, the preloaded Select filter options pass null as the label to Select::isOptionDisabled(), which expects a string — causing a ViewException on /admin/plugin-sales.

Fixes #37

Test plan

• test_plugin_sales_page_renders_when_plugin_has_null_name — verifies the page renders without error when plugins with null names exist
• test_plugin_sales_page_renders_successfully — verifies general page rendering

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