fix(libsinsp): fix crash in string transformers when called on an empty string

[LucaGuerra]

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area libsinsp

Does this PR require a change in the driver versions?

No

What this PR does / why we need it:

Fix a potential crash in string transformers when attempting to read an empty output buffer.
Also, lightly refactor tests and cover the empty string case.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

fix(libsinsp): fix crash in string transformers when called on an empty string

[LucaGuerra]

/milestone 0.18.0

[github-actions]

Perf diff from master - unit tests

     4.72%     -0.85%  [.] sinsp_parser::process_event
     5.50%     -0.80%  [.] next
     3.76%     +0.77%  [.] sinsp_evt::load_params
     1.55%     -0.75%  [.] 0x00000000000e7390
     1.29%     +0.62%  [.] std::_Hashtable<long, std::pair<long const, std::shared_ptr<sinsp_threadinfo> >, std::allocator<std::pair<long const, std::shared_ptr<sinsp_threadinfo> > >, std::__detail::_Select1st, std::equal_to<long>, std::hash<long>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >::_M_find_before_node
     3.04%     -0.53%  [.] std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release
     1.19%     +0.48%  [.] sinsp::fetch_next_event
     0.46%     +0.48%  [.] sinsp_threadinfo::~sinsp_threadinfo
     1.85%     -0.47%  [.] sinsp_evt::get_ts
     0.05%     +0.38%  [.] sinsp_container_info::~sinsp_container_info

Perf diff from master - scap file

     3.78%     +4.39%  [.] sinsp_filter_check::apply_transformers
    11.32%     -2.88%  [.] sinsp_filter_check::tostring
     6.22%     -1.53%  [.] sinsp_parser::process_event
     3.75%     +1.47%  [.] sinsp_filter_check::get_field_info
     5.62%     -1.41%  [.] std::_Hashtable<long, std::pair<long const, std::shared_ptr<sinsp_threadinfo> >, std::allocator<std::pair<long const, std::shared_ptr<sinsp_threadinfo> > >, std::__detail::_Select1st, std::equal_to<long>, std::hash<long>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >::_M_find_before_node
     5.42%     -1.29%  [.] sinsp_evt::get_thread_info
     7.62%     -1.19%  [.] sinsp_thread_manager::find_thread
     3.78%     +0.99%  [.] sinsp_filter_check_thread::extract_single
     7.00%     -0.79%  [.] std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>
     3.78%     -0.61%  [.] gzfile_read

Heap diff from master - unit tests

total runtime: -0.20s.
calls to allocation functions: 652 (-3180/s)
temporary memory allocations: 288 (-1404/s)
peak heap memory consumption: 0B
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

Heap diff from master - scap file

total runtime: 0.01s.
calls to allocation functions: 0 (0/s)
temporary memory allocations: -1 (-71/s)
peak heap memory consumption: 0B
peak RSS (including heaptrack overhead): 0B
total memory leaked: 0B

[poiana]

LGTM label has been added.

Git tree hash: ada1edbc4d215dbac8286ee3c00f82695cf41a9b

[FedeDP]

/approve
Great catch ™️ !

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: FedeDP, jasondellaluce, LucaGuerra

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• OWNERS [FedeDP,LucaGuerra,jasondellaluce]

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[FedeDP]

/milestone 0.17.3