Be more consistent/liberal about stanza accessors.

Return the current value whenever the stanza method is called
without arguments, including when the stanza has no value.  Do
not attempt to construct an instance with no arguments (which
typically throws an exception).

Incidental comment re: depends_on

Refs #7880

lib/cask/dsl.rb

@@ -82,6 +82,7 @@ def homepage(homepage=nil)
     end
 
     def url(*args)
+      return @url if args.empty?
       if @url and !args.empty?
         raise CaskInvalidError.new(self.token, "'url' stanza may only appear once")
       end
@@ -93,6 +94,7 @@ def url(*args)
     end
 
     def appcast(*args)
+      return @appcast if args.empty?
       if @appcast and !args.empty?
         raise CaskInvalidError.new(self.token, "'appcast' stanza may only appear once")
       end
@@ -104,6 +106,7 @@ def appcast(*args)
     end
 
     def gpg(*args)
+      return @gpg if args.empty?
       if @gpg and !args.empty?
         raise CaskInvalidError.new(self.token, "'gpg' stanza may only appear once")
       end
@@ -115,6 +118,7 @@ def gpg(*args)
     end
 
     def container(*args)
+      return @container if args.empty?
       if @container and !args.empty?
         # todo: remove this constraint, and instead merge multiple container stanzas
         raise CaskInvalidError.new(self.token, "'container' stanza may only appear once")
@@ -147,6 +151,7 @@ def version(arg=nil)
     end
 
     def tags(*args)
+      return @tags if args.empty?
       if @tags and !args.empty?
         # consider removing this limitation
         raise CaskInvalidError.new(self.token, "'tags' stanza may only appear once")
@@ -159,6 +164,7 @@ def tags(*args)
     end
 
     def license(arg=nil)
+      return @license if arg.nil?
       if @license and !arg.nil?
         raise CaskInvalidError.new(self.token, "'license' stanza may only appear once")
       end
@@ -169,7 +175,9 @@ def license(arg=nil)
       end
     end
 
+    # depends_on uses a load method so that multiple stanzas can be merged
     def depends_on(*args)
+      return @depends_on if args.empty?
       @depends_on ||= Cask::DSL::DependsOn.new()
       begin
         @depends_on.load(*args) unless args.empty?
@@ -184,6 +192,7 @@ def conflicts_with(*args)
         # todo: remove this constraint, and instead merge multiple conflicts_with stanzas
         raise CaskInvalidError.new(self.token, "'conflicts_with' stanza may only appear once")
       end
+      return @conflicts_with if args.empty?
       @conflicts_with ||= begin
         Cask::DSL::ConflictsWith.new(*args) unless args.empty?
       rescue StandardError => e