stream,net: slightly more consistent typechecking

• stream: remove undefined check

  validChunk allowed undefined as a chunk in object mode; however, this was redundant, since:

  • validChunk() is only used by .write()
  • If the validChunk() check passes for undefined, .write() calls writeOrBuffer()
  • If writeOrBuffer() does not receive a Buffer, it calls decodeChunk()
    • decodeChunk() ignores undefined because it checks typeof chunk === 'string'
    • After that call, chunk.length is accessed, which throws an error if chunk is undefined.

  This “fixes” a bug in the sense that state.pendingcb is no longer incremented for write attempts that fail like this.

• net: remove Socket.prototype.write

  This is superfluous now that typechecking in net and stream are aligned.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included (test-net-connect-buffer checks the error for undefined)
• commit message follows commit guidelines

Affected core subsystem(s)

net, stream