Fix NPE in schema migration for empty fields

[Jotschi]

NPE can happen at

mesh/core/src/main/java/com/gentics/mesh/core/data/schema/impl/FieldTypeChangeImpl.java

Line 229 in f0edc45

return new HtmlFieldImpl().setHTML(typeConverter.toString(oldContent.getFields().getField(fieldName, fieldSchema).getValue()));

when the field container does not yet contain the to be migrated field.

mesh-server      | 20:38:04.880 [] INFO  [vert.x-worker-thread-6] [c.g.m.c.d.j.i.NodeMigrationJobImpl] - Handling node migration request for schema {d7a8c27712304586a8c2771230c586f9} from version {fff5290659934e9eb5290659930e9ec7} to version {aec34c4635884a5f834c4635885a5f2e} for release {795ce1e9b6e1418f9ce1e9b6e1e18f17} in project {44493d95140545a5893d951405b5a5e4}
mesh-server      | 20:38:04.905 [] ERROR [vert.x-worker-thread-6] [c.g.m.c.e.m.n.NodeMigrationHandler] - Error while handling container {8e222d0d6e4d482fa22d0d6e4d282f7c} of node {d729c6c99b394ef5a9c6c99b39def54a} during schema migration.
mesh-server      | java.lang.NullPointerException: null
mesh-server      |      at com.gentics.mesh.core.data.schema.impl.FieldTypeChangeImpl.changeToHtml(FieldTypeChangeImpl.java:229)
mesh-server      |      at com.gentics.mesh.core.data.schema.impl.FieldTypeChangeImpl.createFields(FieldTypeChangeImpl.java:173)
[...]
mesh-server      | 20:38:04.956 [] INFO  [vert.x-worker-thread-6] [c.g.m.c.e.m.AbstractMigrationHandler] - Encountered {4} errors during node migration.
mesh-server      | 20:38:04.971 [] ERROR [vert.x-worker-thread-6] [c.g.m.c.e.m.i.MigrationStatusHandlerImpl] - Error handling migration
mesh-server      | io.reactivex.exceptions.CompositeException: 4 exceptions occurred. 
mesh-server      |      at com.gentics.mesh.core.endpoint.migration.node.NodeMigrationHandler.lambda$migrateNodes$5(NodeMigrationHandler.java:129)
[...]
mesh-server      | 20:38:10.367 [] INFO  [vert.x-worker-thread-13] [i.v.e.w.h.i.LoggerHandlerImpl] - 172.20.0.1 - GET /api/v2/schemas/d7a8c27712304586a8c2771230c586f9?lang=en HTTP/1.1 200 1058 - 4 ms
mesh-server      | 20:38:10.414 [] ERROR [OkHttp http://elasticsearch:9200/...] [c.g.m.s.i.AbstractSearchHandler] - Error while processing search response items
mesh-server      | com.gentics.mesh.core.rest.error.GenericRestException: 400 Bad Request search_error_query params {no such index}
mesh-server      |      at com.gentics.mesh.core.rest.error.Errors.error(Errors.java:86)
[...]
mesh-server      |      at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
mesh-server      |      at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
mesh-server      |      at java.base/java.lang.Thread.run(Unknown Source)
mesh-server      | 20:38:10.415 [] ERROR [OkHttp http://elasticsearch:9200/...] [c

TODO:

• Check why this was not covered by tests
• Fix NPE