CrossDefModel validation should not require any friction specification

[arthurvd]

Describe the bug
Initially reported by @LisaWeijers in #199. When reading a cross section definition that has not any friction specification, the validation currently fails. This is wrong, because D-HYDRO always has default values for the frictionId(s).

To Reproduce
Steps to reproduce the behavior:

1. Take example file crsdef.ini.txt (rename to .ini)
2. Load the file using:

from hydrolib.core.io.crosssection.models import CrossDefModel
crossdef_new = CrossDefModel(modeldir / "crsdef.ini")

3. See error

Exception has occurred: ValidationError
796 validation errors for CrossDefModel
crsdef.ini -> definition -> 1389 -> KBR-1468 -> __root__
  Cross section is missing any friction specification. (type=value_error)

Expected behavior
Validation error should not occur, empty frictionId/Type/Value is allowed.

Version info (please complete the following information):

• OS: Windows (N/A)
• Version: 0.2.0