Fix eclbase_format in ModelConfig to allow to be None

[xjules]

Currently even-though we don't use eclipse it is still defaulted to:

ert/src/ert/config/model_config.py

Line 38 in 975e418

eclbase_format_string: str = DEFAULT_ECLBASE_FORMAT

• variable eclbase_format_string should be renamed summary_file_base_name
• it should default to None

The config option should still use ECLBASE though for reading the value.
The relates to the usage of jobname_format_string which should be updated accordingly