Target: Return null when a transitive property is not defined.

Commit v2.8.11~310^2~1 (Keep track of INCLUDE_DIRECTORIES as a vector of
structs., 2012-11-19) added special case of INCLUDE_DIRECTORIES
for the purpose of origin-tracking of individual entries in the property. It
introduced a bug in that it returned an empty string instead of '0' in the
case that no includes have been set.

Commit v2.8.11~289^2~2 (Handle INTERFACE properties transitively for includes
and defines., 2012-09-23) introduced transitive handling of the property
through the link implementation, together with a whitelist of properties
which would be evaluated transitively. Because of the bug introduced
previously, the 'prop' in TargetPropertyNode is non-null,
meaning that the content (the empty string) would be evaluated as a generator
expression.  This was harmless as the follow-up code was only for 'INTERFACE_'
variants of target properties, so the effect was the same.

Commits v2.8.11~280^2~2 (Keep track of properties used to determine linker
libraries., 2012-11-05) and v2.8.11~280^2~1 (Add API to calculate
link-interface-dependent bool properties or error., 2013-01-06) added a way
to track and report errors on properties which both determine and are
determined by the link implementation.  This was later used in generator
expression evaluation by commit v2.8.11~252^2~2 (Make INTERFACE determined
properties readable in generator expressions., 2013-01-19).  If a property
is unset (null), and the link implementation of the target was not being
evaluated, this commit made it possible to evaluate the property from the
link implementation instead.  If the link implementation was being evaluated,
an empty string was returned from the generator expression evaluation, which
might be later reported as an error.

The above logic was written for 'compatible interface' properties, but in
fact it should have also included other properties.  Because of the
empty-string-instead-of-null bug, this code block is not entered for the
INCLUDE_DIRECTORIES property.  At this point, however, the bug still does
not significantly affect behavior, because the follow-up code is still a
no-op for the INCLUDE_DIRECTORIES property, and an empty string is returned
regardless. Commit v2.8.11~189^2~6 (Use the link information as a source of
compile definitions and includes., 2013-02-12) refactored the logic, but also
without a change in behavior.

Commit v2.8.11~156^2~2 (Expand includes and defines transitively
in 'external' genexes., 2013-02-13) refactored the logic again, this time with
a change of behavior. The INCLUDE_DIRECTORIES property was then mapped to
INTERFACE_INCLUDE_DIRECTORIES during transitive generator expression
evaluation.  Because the transitive evaluation involved evaluation of the
link implementation, this introduced a recursive loop and a segfault with
code like:

  add_library(empty1 ...)
  add_library(empty2 ...)
  target_link_libraries(empty1
    PRIVATE
      $<$<STREQUAL:$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>,/foo/bar>:empty2>
  )

As there is no real use-case for reading a target property like that while
evaluating the link implementation, this went unnoticed.  The same pattern
was followed for other special-cased reads of transitive target properties
such as COMPILE_DEFINITIONS.

The segfault was fixed in the parent commit, but change the property to
return null when appropriate for other future uses.

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