Restrict vulnerability read update for non default branch
This MR skips vulnerability read update when dismissing pipeline security findings which do not exist on the default branch.
Context: We do not create vulnerability read when vulnerability is not present of default branch. That is why currently if we try to update read entry while updating existing state transition it throws an error undefined method `update!' for nil:NilClass
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Related to #428893 (closed)
Edited by Subashis Chakraborty