[MLv2] [BE] [Bug] Support constants in expressions

[uladzimirdev]

Context

Support constants when we add expressions to the query

const constant = 1;
const expressionClause = Lib.expressionClauseForLegacyExpression(query, stageIndex, constant);

const newQuery = Lib.expression(query, stageIndex, "expressionClause", expressionClause)
const expression = Lib.expressions(newQuery, stageIndex)[0]

Lib.legacyExpressionForExpressionClause(query, stageIndex, expression)

// returns ['value', 1, null]

// should be 1

Works fine

const constant = 1;
Lib.legacyExpressionForExpressionClause(query, stageIndex, Lib.expressionClauseForLegacyExpression(query,stageIndex, constant));

// returns 1

[ranquild]

The user-facing issue is described here #37292