Fixed nullable strings (#144)

README.md

charts/mariadb/Chart.yaml

charts/mariadb/README.md

charts/mariadb/values.yaml

charts/mongodb/Chart.yaml

charts/mongodb/README.md

charts/mongodb/values.yaml

charts/postgres/Chart.yaml

charts/postgres/README.md

charts/postgres/values.yaml

charts/rabbitmq/Chart.yaml

charts/rabbitmq/README.md

charts/rabbitmq/values.yaml

charts/redis/Chart.yaml

charts/redis/README.md

charts/redis/values.yaml