Invalid version check doesn't work for Legacy plugins

[mshustov]

Kibana version: master

Describe the bug:
Kibana doesn't disable plugins with invalid version, but crashes.

Steps to reproduce:

1. create a plugin that cannot be used with the current Kibana version. For example, a plugin for 7.4.0
2. start Kibana
3. Kibana crashes

Expected behavior: Kibana starts and logs Plugin "${name}" was disabled because it expected Kibana version "${pluginVersion}", and found "${kibanaVersion}".

Provide logs and/or server output (if relevant):

  log   [16:49:46.009] [fatal][root] TypeError: Cannot read property 'version' of undefined
    at MapSubscriber.version (/Users/mikhailshustov/work/kibana/src/core/server/legacy/plugins/find_legacy_plugin_specs.ts:102:44)

fails on reading from https://github.com/restrry/kibana/blob/3131dd494d623c72690b4a6335f109d19e6d3cc3/src/core/server/legacy/plugins/find_legacy_plugin_specs.ts#L102

Any additional context: can be affected by recent changes #43433

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[MorrieAtElastic]

What is the current status of this problem? Is a code fix planned? If so any idea when it will be available? Thanks

[pgayvallet]

@MorrieAtElastic The issue has been prioritized and should be addressed before 7.7