compiler: bugfix: mishandled empty test

* share/txr/stdlib/compiler.tl (compiler comp-for): Fix
exception thrown when compiling (for init test step ...)
when test is nil. Firstly, we must distinguish a (nil)
test from (), because the latter means (t). Hence the need for
the test-p Boolean. The list of frags must not contain a
nil, which isn't a frag. The instruction template must not
only omit generating the conditional jump when the test is
absent, but also omit generating the test code (insertion of
tfrag.code) in that case, because tfrag is nil.

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