Use `#present?` and `#blank?` to check env variable

Library/Homebrew/cmd/untap.rb

@@ -25,7 +25,7 @@ module Homebrew
     args = untap_args.parse
 
     args.named.to_installed_taps.each do |tap|
-      odie "Untapping #{tap} is not allowed" if tap.core_tap? && !ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+      odie "Untapping #{tap} is not allowed" if tap.core_tap? && ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"].blank?
 
       installed_tap_formulae = Formula.installed.select { |formula| formula.tap == tap }
       installed_tap_casks = Cask::Caskroom.casks.select { |cask| cask.tap == tap }

Library/Homebrew/cmd/update-report.rb

@@ -207,7 +207,7 @@ module Homebrew
 
   def install_core_tap_if_necessary
     return if ENV["HOMEBREW_UPDATE_TEST"]
-    return if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+    return if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"].present?
 
     core_tap = CoreTap.instance
     return if core_tap.installed?

Library/Homebrew/formulary.rb

@@ -361,7 +361,7 @@ module Formulary
     end
 
     def get_formula(*)
-      if !CoreTap.instance.installed? && ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+      if !CoreTap.instance.installed? && ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"].present?
         raise CoreTapFormulaUnavailableError, name
       end
 

Library/Homebrew/tap.rb

@@ -734,7 +734,7 @@ class CoreTap < Tap
 
   def self.ensure_installed!
     return if instance.installed?
-    return if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+    return if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"].present?
 
     safe_system HOMEBREW_BREW_FILE, "tap", instance.name
   end
@@ -751,7 +751,7 @@ class CoreTap < Tap
   # @private
   sig { params(manual: T::Boolean).void }
   def uninstall(manual: false)
-    raise "Tap#uninstall is not available for CoreTap" unless ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"]
+    raise "Tap#uninstall is not available for CoreTap" if ENV["HOMEBREW_UNTAP_HOMEBREW_CORE"].blank?
 
     super
   end