Ninja: Avoid empty phony edges for target ordering

Since commit v3.9.0-rc1~230^2~2 (ninja: break unnecessary target
dependencies, 2017-04-17) we unconditionally generate a phony edge for
target ordering.  It is needed in case a later target depends on it.
However, if the phony edge has no inputs then `ninja -d explain` prints:

    ninja explain: output ... of phony edge with no inputs doesn't exist

Furthermore the phony edge's output is considered dirty and can cause
dependents to be incorrectly considered dirty.  Avoid this by always
generating at least one input to the target ordering phony edges.
If we have no real dependencies just use a path that always exists.

Fixes: #17942

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