fix(baseapp): add nil check for snapshotManager in Commit

[Fallengirl]

Description

snapshotManager.SnapshotIfApplicable() was called without nil check in Commit(), while all other usages in the same file properly check for nil first. This would cause a panic if snapshot manager wasn't configured.

[aljo242]

we require tests for all functionality changes

[Fallengirl]

Added test TestABCI_Commit_WithNilSnapshotManager. Note: SnapshotIfApplicable already has an internal nil check, so this change is technically safe either way. However, this explicit nil check follows the same defensive pattern used elsewhere in Commit() — see Precommiter check on line 1019 and PrepareCheckStater check on line 1055. It makes the nil-safety explicit at the call site rather than relying on the internal implementation.