Remove redundant check from MissingMigrations method

[andrew-farries]

There is no need for this check, as the method will anyway return an empty slice if the schema history is empty.

There is a test for this case here:

pgroll/pkg/roll/missing_test.go

Lines 169 to 185 in 68fc1f6

	t.Run("no migrations have been applied to the target database", func(t *testing.T) {
	fs := fstest.MapFS{
	"01_migration_1.json": &fstest.MapFile{Data: exampleMigJSON(t, "01_migration_1")},
	"02_migration_2.json": &fstest.MapFile{Data: exampleMigJSON(t, "02_migration_2")},
	}
	testutils.WithMigratorAndConnectionToContainer(t, func(m *roll.Roll, _ *sql.DB) {
	ctx := context.Background()
	// Get missing migrations
	migs, err := m.MissingMigrations(ctx, fs)
	require.NoError(t, err)
	// Assert that no migrations are missing from the local directory
	require.Len(t, migs, 0)
	})
	})

Related to #872

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/xataio/pgroll/pkg/roll	79.75% (-0.00%)	👎

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/xataio/pgroll/pkg/roll/missing.go	83.33% (+0.72%)	18 (-5)	15 (-4)	3 (-1)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.