[UI] Remove header when there are not agents on the `Agents` section

[Desvelao]

Wazuh	Splunk	Rev
4.3	8.1.4	4301

Description
A header is displayed when there are not registered agents on the Agents section. The header contains 2 buttons that could be removed:

• Add new agent: Open the deployment agent guide
• Formatted: Export to CSV the table of agents, that for this case, it is not present.

Steps to reproduce

1. Without registered agents, go to the Agents section
2. See the header with the 2 buttons that could not have sense in the UI.

Screenshots