Expression has 8 elements, but must have 9 elements, and also, else null

first of all, you cannot do arithmatic with std_logic_vectors with standard VHDL - to do this you have to include non-standard packages, I assume you've used std_logic_unsigned (not part of the VHDL standard)

To do arithmatic you need to use numeric_std and used the signed/unsigned types instead.

When you do use a standard part for arithmatic (and the non-standard std_logic_signed/unsigned), none of them do bit growth for you - you need to handle that yourself by extending the inputs to the same length as the output before addition. There are two ways of doing this:

For unsigned, append a '0' at the MSB, or for signed, append the sign bit:

Unsigned:

X := ( '0' & A) +( '0' & B ) +( '0' & C) +( '0' & D);

Signed

X := ( A(A'high) & A) + ( B(B'high) & B ) + ( C(C'high) & C) + ( D(D'high) & D);

Or, using the standard library numeric std, you can use the resize function that works for signed or unsigned types:

X := resize(A, X'length) + resize(B, X'length) + resize(C, X'length) + resize(D, X'length);

For your second question, this is legal VHDL. But you have to consider that logic it will generate. If you have this code:

process(condition)
begin
  if condition then
    output <= ip;
  else 
    null;
  end if;
end process;

You are going to generate a latch because you are asking the output to hold it's value when the condition is false. But if you put it in a clocked process, you generate a register with an enable pin, but here the else case is redundant because you get the same behaviour with or without it:

process(clk)
begin
  if rising_edge(clk) then
    if condition then
      output <= ip;
    else
      null;  --redundant, because you get the same behaviour with or without the else case
    end if;
  end if;
end if;

This shows the inportance of understanding what logic your code is going to generate.

For synthesisable code, null is hardly ever used because of the redundancy or undesirable logic shown above. But it can be used to clarify that you explicity want to do nothing and havent just forgotten to fill the code it (for example wait states in a state machine).

Null does have far more uses in complex test benches, as it is the default value for pointers.