* PHP-7.2:
  Fix #78694: Appending to a variant array causes segfault

`write_dimension` object handlers have to be able to handle `NULL`
`offset`s; for now we simply throw an exception instead of following
the `NULL` pointer.

No need to define our own `com_dotnet_object_properties` as empty hash,
since we can use `zend_empty_array` instead as of PHP 7.3.0.

Cf. <https://github.com/php/php-src/pull/3672#issuecomment-440467704>.

* PHP-7.2:
  Fix #77177: Serializing or unserializing COM objects crashes

Firstly, we avoid returning NULL from the get_property handler, but
instead return an empty HashTable, which already prevents the crashes.
Secondly, since (de-)serialization obviously makes no sense for COM,
DOTNET and VARIANT objects (at least with the current implementation),
we prohibit it right away.

The $Id$ keywords were used in Subversion where they can be substituted
with filename, last revision number change, last changed date, and last
user who changed it.

In Git this functionality is different and can be done with Git attribute
ident. These need to be defined manually for each file in the
.gitattributes file and are afterwards replaced with 40-character
hexadecimal blob object name which is based only on the particular file
contents.

This patch simplifies handling of $Id$ keywords by removing them since
they are not used anymore.

convert_scalar_to_number() will now call cast_object() with an
_IS_NUMBER argument, in which case the cast handler should return
either an integer or floating point number, whichever is more
appropriate.

Previously convert_scalar_to_number() unconditionally converted
objects to integers instead.

Fixes bug #53033.
Fixes bug #54973.
Fixes bug #73108.