exp run: fails when empty string or nothing is supplied

[aguschin]

Bug Report

Description

When I try to run experiment and set some parameter to be empty or "", I get an error.

Reproduce

dvc exp run -S param=
ERROR: Must provide a value for parameter 'param'
# or
dvc exp run -S param=""
ERROR: Must provide a value for parameter 'param'

Expected

For the dvc exp run -S param= I expected to get the following:

# params.yaml
param:

And for the dvc exp run -S param="" I expected to get:

# params.yaml
param: ""

Environment information

Checked this few days ago on dvc installed from the commit in master branch 2.3.0+eb46c0

[aguschin]

@skshetry, created an issue, as you requested #6107 (comment)