remove null or empty values which were parsed in json in the alert dialog

Welcome to so. You defined the default value as ‘0’ in Tb in your model. Your UserDetails model should be like:

Tb:json['Tb'],

And then, you should check the Tb value is null. Like this:

              content: _userDetails[index].Tb != null
                      ? Text('Formül: ' + _userDetails[index].Formula + '\nTfp: ' + _userDetails[index].Tfp +'\nTb: ' + _userDetails[index].Tb +'\nTc: ' + _userDetails[index].Tc +'\nPc: ' + _userDetails[index].Pc +'\nVc: ' + _userDetails[index].Vc +'\nZc: ' + _userDetails[index].Zc +'\nΩ: ' + _userDetails[index].Omega +'\nDipm: ' + _userDetails[index].Dipm +'\nCpA: ' + _userDetails[index].CpA+'\nCpB: ' + _userDetails[index].CpB +'\nCpC: ' + _userDetails[index].CpC +'\nCpD: ' + _userDetails[index].CpD +'\ndHf: ' + _userDetails[index].dHf +'\ndGf: ' + _userDetails[index].dGf +'\nEq: ' + _userDetails[index].Eq +'\nLden: ' + _userDetails[index].Lden +'\nTden: ' + _userDetails[index].Tden)
                      : Text('Formül: ' + _userDetails[index].Formula + '\nTfp: ' + _userDetails[index].Tfp + '\nTc: ' + _userDetails[index].Tc +'\nPc: ' + _userDetails[index].Pc +'\nVc: ' + _userDetails[index].Vc +'\nZc: ' + _userDetails[index].Zc +'\nΩ: ' + _userDetails[index].Omega +'\nDipm: ' + _userDetails[index].Dipm +'\nCpA: ' + _userDetails[index].CpA+'\nCpB: ' + _userDetails[index].CpB +'\nCpC: ' + _userDetails[index].CpC +'\nCpD: ' + _userDetails[index].CpD +'\ndHf: ' + _userDetails[index].dHf +'\ndGf: ' + _userDetails[index].dGf +'\nEq: ' + _userDetails[index].Eq +'\nLden: ' + _userDetails[index].Lden +'\nTden: ' + _userDetails[index].Tden),

EDIT:

IMHO, there is no easy way without RichText.

RichText(
    text: TextSpan(
        children: <TextSpan>[
          _userDetails[index].Formula != null ? TextSpan(text: 'Formül: ' + _userDetails[index].Formula,) : null,
          _userDetails[index].Tfp ! = null ? TextSpan(text: '\nTfp: ' + _userDetails[index].Tfp) : null,
          _userDetails[index].Tb != null ? TextSpan(text: '\nTb: ' + _userDetails[index].Tb) : null
        ],
    ),
  )