Merge branch 'fix-amg-empty-matrix-on-one-proc' into 'master'

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

See merge request !159 (merged)

(cherry picked from commit 0ad9d513)

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

enabled an automatic merge when the pipeline for ecc4a503 succeeds

mentioned in commit eaaedbe6

merged