MINOR: htx: never check for null htx pointer in htx_is_{,not_}empty()

MINOR: htx: never check for null htx pointer in htx_is_{,not_}empty()

The previous patch clarifies the fact that the htx pointer is never null
along all the code. This test for a null will never match, didn't catch
the pointer 1 before the fix for b_is_null(), but it confuses the compiler
letting it think that any dereferences made to this pointer after this
test could actually mean we're dereferencing a null. Let's now drop this
test. This saves us from having to add impossible tests everywhere to
avoid the warning.

This should be backported to 1.9 if the b_is_null() patch is backported.