evalengine bugfix: handle nil evals correctly when coercing values

[systay]

Description

When coercing values to a common type in UNION, we hit a panic for BLOB values.

Related Issue(s)

Fixes #14907

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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[dbussink]

Nice! Can you also add a unit test here?

vitess/go/vt/vtgate/evalengine/api_arithmetic_test.go

Lines 738 to 749 in ab76091

	}, {
	// For float, we should not use scientific notation.
	typ: sqltypes.Decimal,
	v: newEvalFloat(1.2e-16),
	out: TestValue(sqltypes.Decimal, "0.00000000000000012"),
	}}
	for _, tcase := range tcases {
	got := evalToSQLValueWithType(tcase.v, tcase.typ)
	if !reflect.DeepEqual(got, tcase.out) {
	t.Errorf("toSQLValue(%v, %v): %v, want %v", tcase.v, tcase.typ, printValue(got), printValue(tcase.out))
	}
	}

[dbussink]

Good to go once the unit test is added.