First thanks to user30184 who helped me to continue searching… I found a solution

in the layer METADATA I added two metadatas: "wcs_rangeset_nullvalue" "-99999"wich alone does no do anything but -99999 is my geoTiff "nodata" real value (in the tiff metadata there is a tag nodata=-99999). For working it needs also "wcs_rangeset_name" "DEM 75m" , without this no WCS are sent to the client.

This is mu complete and correct map file:

MAP
  OUTPUTFORMAT
      NAME "GTiff"
      DRIVER GDAL/GTiff
      MIMETYPE "image/tiff"
      IMAGEMODE FLOAT32
      EXTENSION "tif"
    TRANSPARENT ON
  END

  NAME "Modèle numérique de terrain"
    EXTENT  -6.0001389 39.9998611 10.0001389 52.0001389
    MAXSIZE 4096
    PROJECTION
        "init=epsg:2154"
  END

  WEB
    METADATA
        "wcs_label"           "WCS Server"
        "wcs_onlineresource"  "http://localhost/cgi-bin/mapserv?map=/tmp/testNaN.map&"
        "wcs_enable_request" "*"
            "wcs_fees"            "none"
            "wcs_accessconstraints"    "none"
            "wcs_keywordlist"          "wcs,dem"
            "wcs_metadatalink_type"    "TC211"
            "wcs_metadatalink_format"  "text/plain"
            "wcs_metadatalink_href"    "http://localhost"
            "wcs_address"              ""
            "wcs_city"                 "Paris"
            "wcs_stateorprovince"      "IDF"
            "wcs_postcode"             "75000"
            "wcs_country"              "France"
            "wcs_contactelectronicmailaddress" "[email protected]"
            "wcs_contactperson"            "me"
            "wcs_contactorganization"      "unemployed"
            "wcs_contactposition"          "manager"
            "wcs_contactvoicetelephone"    ""
            "wcs_contactfacimiletelephone" ""
            "wms_title"           "WMS Server"
            "wms_onlineresource"  "http://localhost/cgi-bin/mapserv?map=/tmp/testNaN.map&"
            "wms_srs"             "EPSG:2154"
            "wms_enable_request"  "*"
            "wfs_title"          "WFS Server" 
            "wfs_onlineresource" "http://localhost/cgi-bin/mapserv?map=/tmp/testNaN.map&" ## Recommended
            "wfs_srs"            "EPSG:2154 EPSG:4326 EPSG:5698" ## Recommended
            "wfs_abstract"       "WFS Server" 
            "wfs_enable_request" "*"  
      END
    END

  LAYER
        NAME TESTNAN
          DATA "/tmp/outputNaN.tif"
        STATUS ON
        TYPE RASTER
        PROCESSING "SCALE=AUTO"
        PROJECTION
           "init=epsg:2154"
        END
    METADATA
      "wms_title"         "DEM 75m"
      "wcs_description" "Modèle numérique de terrain"
      "wcs_name" "DEM 75m"
      "wcs_label" "DEM (rés: 75m)"
      "wcs_srs" "EPSG:2154"
      "wcs_enable_request" "*"
      "wcs_bandcount" "1"
      "wcs_band_names" "Altitude"
      "Altitude_band_description" "Altitude en mètres"
      "wcs_rangeset_nullvalue" "-99999"
      "wcs_rangeset_name" "DEM 75m"      
    END
  END
END