case when evaluating else condition anyway?

I've

case when ''=extinput then null else extinput::timestamp end

now when extinput='' it seems that else extinput::timestamp get
evaluated anyway and I get:

invalid input syntax for type timestamp: ""

The purpose of all this gymnic would be to convert empty string to
null and everything else to a timestamp.

Is there any cleaner functional way that doesn't involve prepared
statement etc... since the whole exercise is caused by an
null-impaired DB API (aka MySQLish).

--
Ivan Sergio Borgonovo
http://www.webthatworks.it