Reduced the number of function calls required for .find() (single element...

Reduced the number of function calls required for .find() (single element root), optimized some calls to jQuery() as well. Goes towards fixing #4240.

src/core.js

@@ -22,7 +22,16 @@ var
 jQuery.fn = jQuery.prototype = {
 	init: function( selector, context ) {
 		// Make sure that a selection was provided
-		selector = selector || document;
+		if ( selector === undefined ) {
+			selector = document;
+		}
+
+		// Handle "", null
+		if ( !selector ) {
+			this.length = 0;
+			this.context = document;
+			return this;
+		}
 
 		// Handle $(DOMElement)
 		if ( selector.nodeType ) {
@@ -31,6 +40,7 @@ jQuery.fn = jQuery.prototype = {
 			this.context = selector;
 			return this;
 		}
+
 		// Handle HTML strings
 		if ( typeof selector === "string" ) {
 			// Are we dealing with HTML string or an ID?
@@ -40,17 +50,18 @@ jQuery.fn = jQuery.prototype = {
 			if ( match && (match[1] || !context) ) {
 
 				// HANDLE: $(html) -> $(array)
-				if ( match[1] )
+				if ( match[1] ) {
 					selector = jQuery.clean( [ match[1] ], context );
 
 				// HANDLE: $("#id")
-				else {
+				} else {
 					var elem = document.getElementById( match[3] );
 
 					// Handle the case where IE and Opera return items
 					// by name instead of ID
-					if ( elem && elem.id != match[3] )
+					if ( elem && elem.id != match[3] ) {
 						return jQuery().find( selector );
+					}
 
 					// Otherwise, we inject the element directly into the jQuery object
 					var ret = jQuery( elem || [] );
@@ -61,13 +72,21 @@ jQuery.fn = jQuery.prototype = {
 
 			// HANDLE: $(expr, [context])
 			// (which is just equivalent to: $(content).find(expr)
-			} else
-				return jQuery( context ).find( selector );
+			} else if ( !context || context.nodeType ) {
+				this[0] = context || document;
+				this.length = 1;
+				this.context = context;
+				return this.find( selector );
+
+			} else {
+				return (context.jquery ? context : jQuery( context )).find( selector );
+			}
 
 		// HANDLE: $(function)
 		// Shortcut for document ready
-		} else if ( jQuery.isFunction( selector ) )
+		} else if ( jQuery.isFunction( selector ) ) {
 			return jQuery( document ).ready( selector );
+		}
 
 		// Make sure that old selector state is passed along
 		if ( selector.selector && selector.context ) {
@@ -274,8 +293,7 @@ jQuery.fn = jQuery.prototype = {
 
 	find: function( selector ) {
 		if ( this.length === 1 ) {
-			var ret = this.pushStack( [], "find", selector );
-			ret.length = 0;
+			var ret = this.pushStack( "", "find", selector );
 			jQuery.find( selector, this[0], ret );
 			return ret;
 		} else {