Issue Title: Error Computing String Metric in Column - Value Type Error

[Youen Péron]

Issue Description:
When processing the provided JSON input data, the Rimo tool encounters an error that leads to the program's failure. The error message indicates an issue with computing a string metric in the "PRENOM" column, resulting in a value type error. This error seems to be related to the data type or content within the "PRENOM" column.

Error Details:

1:41PM INF rimo v0.1.0 (commit=391e8980ca365942c6b30e3993a73902eade156a date=2023-08-23 by=youen)
2023/08/23 13:41:56 failed to compute metric: error computing string metric in column PRENOM : value type error : expected numeric found <nil>: <nil>

Steps to Reproduce:

1. Use the provided JSON input data:

   {"EMAIL":"[e-mail]","GROUPE_ID":3,"ID":61,"ID_TYPE_UTILISATEUR":null,"LOGIN":"*****","MDP":"******","NOM":"***","PRENOM":null,"PROFIL_ID":2}

2. Run the Rimo tool with the data.

Expected Behavior:
The Rimo tool should be able to process the input data without encountering a value type error in the "PRENOM" column. The program should successfully compute the relevant metrics and perform the necessary operations without failures.

Actual Behavior:
The tool encounters a value type error in the "PRENOM" column, leading to the program's failure. This issue prevents the program from completing its execution successfully.

Additional Context:

• Rimo version: [Specify Rimo version here]
• Operating System: [Specify your operating system here]
• This issue seems to be related to a specific column ("PRENOM") within the input data. The column might contain null values or other data that is causing the type error during metric computation.

[Mathis Derenne]

Fixed in refactor MR.