Fix segmentation fault in AMG if matrix of one rank is empty.

For this we need to mark a matrix without rows built in row_wise mode as built once the CreateIterator is constructed and correct the number of vertices for a Subgraph of a MatrixGraph with 0 vertices.

This should fix the segementatio fault Kumar is experiencing.

Splitted into 2 logical commits.

added 2 commits

• 1b126e17 - Fix segmentation fault in AMG if matrix of one rank is empty.
• 8434134d - Make BCRSMatrix::createBegin mark empty matrices as built.

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enabled an automatic merge when the pipeline for 8434134d succeeds

mentioned in commit 0ad9d513

merged

mentioned in commit ecc4a503

mentioned in merge request !160 (merged)