Remove duplicate null checks

Author: oozcitak

src/XMLAttribute.coffee

@@ -14,9 +14,8 @@ module.exports = class XMLAttribute
 
     if not name?
       throw new Error "Missing attribute name"
-
-    # evaluate if function
-    value = value.apply() if _.isFunction value
+    if not value?
+      throw new Error "Missing attribute value"
 
     @name = @stringify.attName name
     @value = @stringify.attValue value

src/XMLElement.coffee

@@ -37,7 +37,7 @@ module.exports = class XMLElement extends XMLNode
     clonedSelf = _.clone @
 
     # clone attributes
-    clonedSelf.attributes = {}    
+    clonedSelf.attributes = {}
     for own attName, att of @attributes
       clonedSelf.attributes[attName] = _.clone att
 
@@ -62,14 +62,8 @@ module.exports = class XMLElement extends XMLNode
   # `name` attribute name
   # `value` attribute value
   attribute: (name, value) ->
-    if not name?
-      throw new Error "Missing attribute name"
-    if not value?
-      throw new Error "Missing attribute value"
-
-    @attributes ?= {}
+    value = value.apply() if _.isFunction value
     @attributes[name] = new XMLAttribute @, name, value
-
     return @
 
 
@@ -90,6 +84,7 @@ module.exports = class XMLElement extends XMLNode
   # `target` instruction target
   # `value` instruction value
   instruction: (target, value) ->
+    value = value.apply() if _.isFunction value
     instruction = new XMLProcessingInstruction @, target, value
     @instructions.push instruction
     return @

src/XMLProcessingInstruction.coffee

@@ -15,9 +15,6 @@ module.exports = class XMLProcessingInstruction
     if not target?
       throw new Error "Missing instruction target"
 
-    # evaluate if function
-    value = value.apply() if _.isFunction value
-
     @target = @stringify.insTarget target
     @value = @stringify.insValue value if value