.show() and .hide() throw errors with falsy values for speed

[scott.gonzalez]

Calling .show(null) used to behave the same as .show(), but now throw errors. Same for .hide().

Fixes:
 http://github.com/scottgonzalez/jquery/commit/2d2e626aa2e4956f33e8b03bb0ee624267d53c22
 http://github.com/scottgonzalez/jquery/commit/e04edb260a201586bab3c51d5eda6225a5e8c5c5

[scott.gonzalez]

Test:  http://github.com/scottgonzalez/jquery/commit/22b6c9667637df2f2b8429542050198768b915f1

[john]

Landed.  http://github.com/jquery/jquery/commit/231c8756788f085d04c6120c12eaa66766abfa3f  http://github.com/jquery/jquery/commit/9cd5866a292b1d653651237802d80393a9081528  http://github.com/jquery/jquery/commit/8cb8ae27d8e0c732b711d73d2ef71125b0a7769b