Fix empty_fcall_info C++ missing-field-initializers warning

[KaseyJenkins]

This commit introduced the following change:

#ifndef __cplusplus
# define empty_fcall_info (zend_fcall_info) {0}
# define empty_fcall_info_cache (zend_fcall_info_cache) {0}
#else
# define empty_fcall_info zend_fcall_info {0}
# define empty_fcall_info_cache zend_fcall_info_cache {0}
#endif

When it comes to C++ it generates a -Wmissing-field-initializers warning, which is different to C, where this option does not warn about the universal zero initializer ‘{ 0 }’.

To sort this issue out while still maintaining the expected (compiler) behaviour the empty { } initializer should be employed instead:

#ifndef __cplusplus
# define empty_fcall_info (zend_fcall_info) {0}
# define empty_fcall_info_cache (zend_fcall_info_cache) {0}
#else
# define empty_fcall_info zend_fcall_info {}
# define empty_fcall_info_cache zend_fcall_info_cache {}
#endif

This is particularly important since -Wmissing-field-initializers is included in -Wextra, which, coupled with -Werror precludes a successful C++ extension build.

[iluuu1994]

Thank you @KaseyJenkins!