Below are 3 different solutions for 3 different glitches :
Value 1
1234
blank cell
1234
Solution 2
=IIF(IsNumeric(Fields!TEST.Value.ToString()), CDBL(IIF(IsNumeric(Fields!TEST.Value.ToString()), Fields!TEST.Value.ToString(), "0")), " ")
Value 2
1234
Null
345
null
Solution 2
=IIF(IsNumeric(Fields!TEST.Value.ToString()), CDBL(IIF(IsNumeric(Fields!TEST.Value.ToString()), Fields!TEST.Value.ToString(), "0")), "NULL")
Value 3
1234
Null
345
N/A
550
TBA
440
599
NOT APPLICABLE
Solution 3
=IIF(IsNumeric(Fields!TEST.Value.ToString()), CDBL(IIF(IsNumeric(Fields!TEST.Value.ToString()), Fields!TEST.Value.ToString(), "0")), Fields!TEST.Value.ToString())