Formula: handle bad tap runtime dependencies.

Also add some better exceptions for the cases that `nil`s end up
getting passed around incorrectly.

Library/Homebrew/dependency.rb

@@ -10,6 +10,8 @@ class Dependency
   DEFAULT_ENV_PROC = proc {}
 
   def initialize(name, tags = [], env_proc = DEFAULT_ENV_PROC, option_names = [name])
+    raise ArgumentError, "Dependency must have a name!" unless name
+
     @name = name
     @tags = tags
     @env_proc = env_proc

Library/Homebrew/formula.rb

@@ -1522,7 +1522,10 @@ class Formula
     if read_from_tab &&
        (keg = opt_or_installed_prefix_keg) &&
        (tab_deps = keg.runtime_dependencies)
-      return tab_deps.map { |d| Dependency.new d["full_name"] }.compact
+      return tab_deps.map do |d|
+        next unless d["full_name"]
+        Dependency.new d["full_name"]
+      end.compact
     end
 
     declared_runtime_dependencies | undeclared_runtime_dependencies

Library/Homebrew/formulary.rb

@@ -277,6 +277,7 @@ module Formulary
   # * a formula URL
   # * a local bottle reference
   def self.factory(ref, spec = :stable, alias_path: nil, from: nil)
+    raise ArgumentError, "Formulae must have a ref!" unless ref
     loader_for(ref, from: from).get_formula(spec, alias_path: alias_path)
   end