[Intelligent code completion] Do not log an error when preceding `assign`ed value is not mentioned in the docs

[Poitrin]

Describe the bug :+1: When using e.g. …

• attributes that are clearly defined in the docs, e.g. product.images.first.height
• literal numbers

… suggestions work and no error appears:

:-1: However, when using values that are not explicitly mentioned in the docs, e.g. …

• custom settings like section.settings.columns_mobile
• calculated values like product.variants.size
• unknown values like product_variant.store_availabilities, which are mentioned in the soon-to-be-removed objects.yml, but nowhere in the docs (objects.json)
  • Also mentioned in https://github.com/Shopify/theme-check/pull/674#discussion_r1040636881

… only default suggestions are shown, and when adding/removing characters, errors might happen:

{%- liquid
  assign columns_mobile_int = section.settings.columns_mobile
  assign show_mobile_slider = false
-%}

{%- if show_mobile_slider

Expected No matter the assigned value, code completion should not crash.

Actual When certain values are assigned, code completion crashes.

Stack trace

[completion error] error: undefined method `name' for nil:NilClass
/lib/theme_check/language_server/completion_providers/assignments_completion_provider.rb:19:in `block in completions'
/lib/theme_check/language_server/completion_providers/assignments_completion_provider.rb:17:in `each'
/lib/theme_check/language_server/completion_providers/assignments_completion_provider.rb:17:in `map'
/lib/theme_check/language_server/completion_providers/assignments_completion_provider.rb:17:in `completions'
/lib/theme_check/language_server/completion_engine.rb:16:in `block in completions'
/lib/theme_check/language_server/completion_engine.rb:16:in `each'
/lib/theme_check/language_server/completion_engine.rb:16:in `flat_map'
/lib/theme_check/language_server/completion_engine.rb:16:in `completions'
/lib/theme_check/language_server/handler.rb:145:in `on_text_document_completion'
/lib/theme_check/language_server/server.rb:138:in `handle_message'
/lib/theme_check/language_server/server.rb:109:in `block in handle_messages'
/lib/theme_check/language_server/server.rb:105:in `loop'
/lib/theme_check/language_server/server.rb:105:in `handle_messages'
/lib/theme_check/language_server/server.rb:99:in `block (2 levels) in start_handler_threads'

Debugging information

• OS: macOS
• Version: 1.12.0