Ruby style changes: replace `== 0` with `zero?`

src/ruby/bin/math_server.rb

@@ -55,7 +55,7 @@ class Fibber
     return enum_for(:generator) unless block_given?
     idx, current, previous = 0, 1, 1
     until idx == @limit
-      if idx == 0 || idx == 1
+      if idx.zero? || idx == 1
         yield Math::Num.new(num: 1)
         idx += 1
         next
@@ -94,7 +94,7 @@ end
 # package. That practice should be avoided by defining real services.
 class Calculator < Math::Math::Service
   def div(div_args, _call)
-    if div_args.divisor == 0
+    if div_args.divisor.zero?
       # To send non-OK status handlers raise a StatusError with the code and
       # and detail they want sent as a Status.
       fail GRPC::StatusError.new(GRPC::Status::INVALID_ARGUMENT,

src/ruby/lib/grpc/core/time_consts.rb

@@ -58,7 +58,7 @@ module GRPC
                "Cannot make an absolute deadline from #{timeish.inspect}")
         elsif timeish < 0
           TimeConsts::INFINITE_FUTURE
-        elsif timeish == 0
+        elsif timeish.zero?
           TimeConsts::ZERO
         else
           Time.now + timeish

src/ruby/lib/grpc/generic/rpc_server.rb

@@ -146,7 +146,7 @@ module GRPC
     def remove_current_thread
       @stop_mutex.synchronize do
         @workers.delete(Thread.current)
-        @stop_cond.signal if @workers.size == 0
+        @stop_cond.signal if @workers.size.zero?
       end
     end
 
@@ -364,7 +364,7 @@ module GRPC
     # - #running? returns true after this is called, until #stop cause the
     #   the server to stop.
     def run
-      if rpc_descs.size == 0
+      if rpc_descs.size.zero?
         logger.warn('did not run as no services were present')
         return
       end
@@ -455,7 +455,7 @@ module GRPC
       unless cls.include?(GenericService)
         fail "#{cls} must 'include GenericService'"
       end
-      if cls.rpc_descs.size == 0
+      if cls.rpc_descs.size.zero?
         fail "#{cls} should specify some rpc descriptions"
       end
       cls.assert_rpc_descs_have_methods