<!--- This environment context section helps us quickly review your PR. 
      Please take a minute to fill-out this information. -->

## Environment
**Liquibase Version**: 3.8.8

**Liquibase Integration & Version**: gradle (for a Spring Boot project)

**Liquibase Extension(s) & Version**: N/A

**Database Vendor & Version**: PostgreSQL 9.6.12

**Operating System Type & Version**:  macOS Mojave 10.14.6

## Description
When creating a Liquibase changeset for a Postgres database, including the `loadUpdateData` change type with an attribute of `onlyUpdate=true` within the changeset will still insert any records that were not found. The `onlyUpdate=true` flag should prevent this behavior by only updating existing records.

## Steps To Reproduce
Simple steps to reproduce the problem would be to do the following:

1. Create a Postgres database with an empty table.
1. Create a Spring Boot project with Liquibase version 3.8.8.
1. Create a CSV file to import seed data into the table from step 1.
1. Create a changeset and use `loadUpdateData` with `onlyUpdate=true` to import the seed data from the CSV file into the Postgres table.

## Actual Behavior
Liquibase will insert **all** records from the CSV file into the table even though we specified `onlyUpdate=true`. If your CSV file did not include an `id` column, Liquibase will throw a database exception because id cannot be null.

## Expected/Desired Behavior
Liquibase should not insert any records into the table because we specified `onlyUpdate=true`.

## Additional Context
1. I did some research into this bug and the cause appears to be in the following file: https://github.com/liquibase/liquibase/blob/master/liquibase-core/src/main/java/liquibase/sqlgenerator/core/InsertOrUpdateGeneratorPostgres.java

    Specifically, the `generateSql` method is overriden for the Postgres case but there is no check for `onlyUpdate=true` inside the method. I would expect a conditional check around the `IF not found THEN` block of code to prevent the insert statement from being appended.1. The OS type should not matter for this issue since it's specific to Liquibase.
package liquibase.sqlgenerator.core;
import liquibase.change.ColumnConfig;
import liquibase.database.core.PostgresDatabase;
import liquibase.sql.Sql;
import liquibase.statement.SequenceNextValueFunction;
import liquibase.statement.core.InsertStatement;
import org.junit.Test;
import static org.junit.Assert.assertEquals;
public class InsertOrUpdateGeneratorPostgresTest {
    private static final String CATALOG_NAME = "mycatalog";
    private static final String SCHEMA_NAME = "myschema";
    private static final String TABLE_NAME = "mytable";
    private static final String SEQUENCE_NAME = "my_sequence";
    @Test
    public void testInsertSequenceValWithSchema(){
        PostgresDatabase postgresDatabase = new PostgresDatabase();
        InsertGenerator generator = new InsertGenerator();
        InsertStatement statement = new InsertStatement(CATALOG_NAME, SCHEMA_NAME, TABLE_NAME);
        ColumnConfig columnConfig = new ColumnConfig();
        columnConfig.setValueSequenceNext(new SequenceNextValueFunction(SEQUENCE_NAME, SCHEMA_NAME));
        columnConfig.setName("col3");
        statement.addColumn(columnConfig);
        Sql[] sql = generator.generateSql( statement, postgresDatabase,  null);
        String theSql = sql[0].toSql();
        assertEquals(String.format("INSERT INTO %s.%s (col3) VALUES (nextval('%s.%s'))",SCHEMA_NAME,TABLE_NAME,SCHEMA_NAME,SEQUENCE_NAME)
                ,theSql);
    }
}