Handle nil urls in cask installer and API cask loader

The goal here is to handle the case where a cask might have a nil
url stanza because that cask is not available on the current version
of macOS or the given architecture. This just moves those checks
from the end of the `Cask::Installer#fetch` method to the beginning
so that we don't try and download casks that are missing urls.

This will now provide a helpful error message like so:
```
Error: This software does not run on macOS versions older than Big Sur.
```

Beyond that it no longer tries to run the url stanza with a nil value
when loading casks from the API.

Library/Homebrew/cask/cask_loader.rb

@@ -277,7 +277,7 @@ module Cask
             sha256 json_cask[:sha256]
           end
 
-          url json_cask[:url], **json_cask.fetch(:url_specs, {})
+          url json_cask[:url], **json_cask.fetch(:url_specs, {}) if json_cask[:url].present?
           appcast json_cask[:appcast] if json_cask[:appcast].present?
           json_cask[:name].each do |cask_name|
             name cask_name

Library/Homebrew/cask/installer.rb

@@ -64,12 +64,12 @@ module Cask
       odebug "Cask::Installer#fetch"
 
       load_cask_from_source_api! if @cask.loaded_from_api? && @cask.caskfile_only?
-
       verify_has_sha if require_sha? && !force?
+      check_requirements
 
       download(quiet: quiet, timeout: timeout)
 
-      satisfy_dependencies
+      satisfy_cask_and_formula_dependencies
     end
 
     def stage
@@ -253,25 +253,19 @@ on_request: true)
       end
     end
 
-    # TODO: move dependencies to a separate class,
+    def check_requirements
-    #       dependencies should also apply for `brew cask stage`,
+      check_macos_requirements
-    #       override dependencies with `--force` or perhaps `--force-deps`
+      check_arch_requirements
-    def satisfy_dependencies
-      return unless @cask.depends_on
-
-      macos_dependencies
-      arch_dependencies
-      cask_and_formula_dependencies
     end
 
-    def macos_dependencies
+    def check_macos_requirements
       return unless @cask.depends_on.macos
       return if @cask.depends_on.macos.satisfied?
 
       raise CaskError, @cask.depends_on.macos.message(type: :cask)
     end
 
-    def arch_dependencies
+    def check_arch_requirements
       return if @cask.depends_on.arch.nil?
 
       @current_arch ||= { type: Hardware::CPU.type, bits: Hardware::CPU.bits }
@@ -286,7 +280,7 @@ on_request: true)
             "but you are running #{@current_arch}."
     end
 
-    def collect_cask_and_formula_dependencies
+    def cask_and_formula_dependencies
       return @cask_and_formula_dependencies if @cask_and_formula_dependencies
 
       graph = ::Utils::TopologicalHash.graph_package_dependencies(@cask)
@@ -305,7 +299,7 @@ on_request: true)
     end
 
     def missing_cask_and_formula_dependencies
-      collect_cask_and_formula_dependencies.reject do |cask_or_formula|
+      cask_and_formula_dependencies.reject do |cask_or_formula|
         installed = if cask_or_formula.respond_to?(:any_version_installed?)
           cask_or_formula.any_version_installed?
         else
@@ -315,10 +309,13 @@ on_request: true)
       end
     end
 
-    def cask_and_formula_dependencies
+    # TODO: move dependencies to a separate class,
+    #       dependencies should also apply for `brew cask stage`,
+    #       override dependencies with `--force` or perhaps `--force-deps`
+    def satisfy_cask_and_formula_dependencies
       return if installed_as_dependency?
 
-      formulae_and_casks = collect_cask_and_formula_dependencies
+      formulae_and_casks = cask_and_formula_dependencies
 
       return if formulae_and_casks.empty?